I have a similiar requirement but, unfortuantly, I couldn't accept "no" as an answer as any other solution would have taken just as long and probably entailed more risk and money.  Therefore I created a couple of JavaScript functions (think of them as being equilvent to extension methods in C#).

        /// <summary>
        /// Removes all rows and columns from a grid, effectively reseting it to a completely new grid.  This is useful
        /// for then dynmically re-populating the grid.
        ///
        /// NOTE: THIS CODE IS TIGHTLY BOUND TO THE INFRAGISTICS LIBRARY.  CHANGES TO THE INFRAGISTICS LIBRARY COULD RENDER
        /// THIS CODE INVALID.
        ///
        /// TODO: TEST THIS CODE WHEN UPGRADES ARE MADE TO THE INFRAGISTICS LIBRARY.
        ///
        /// </summary>
        /// <example>igtbl_resetGrid('<%= TestGrid.ID %>');</example>
        /// <param name="gridClientID">The ClientID of the Infragistics grid to be reset</param>
        function igtbl_resetGrid(gridClientID)
        {
            var grid = igtbl_getGridById(gridClientID);
           
            if(grid != null)
            {
                var gridHasRowSelectors = (grid.RowSelectors == 1);                 // Determine if the first column is the row selector column
                var endElementPos = (gridHasRowSelectors == true ? 1 : 0);          // If yes then we want to leave this
               
                /* Remove all the rows first */
                var rowCount = grid.Rows.length;
                var rowCounter = rowCount - 1;
               
                for(rowCounter; rowCounter >= 0; rowCounter--)
                {
                    try
                    {
                        grid.Rows.remove(rowCounter, false);                            // false signifies that we do not want to fire events
                    }
                    catch(e)
                    {
                        // An error is thrown using FireFox, but the row still gets removed.  Hence
                        // we just need to catch it but do nothing with it.
                    }
                }
               
                /* Now remove the column headings by manipulating the DOM... */
                var tableDiv = grid.getDivElement();
                var table = tableDiv.childNodes[0];                                 // Move to the TABLE element which represents the actual table
                var childElementCounter = 0;
                var childElements = table.childNodes.length;
               
                var removeTableElementsDelegate = function(parentElement)
                    {
                        // Iterate over each child element and remove it from the DOM.
                        var elementCounter = parentElement.childNodes.length - 1;
                       
                        for(elementCounter; elementCounter >= endElementPos; elementCounter --)
                        {
                            parentElement.removeChild(parentElement.childNodes[elementCounter]);
                        }
                    };
               
                for(childElementCounter; childElementCounter < childElements; childElementCounter ++)
                {
                    switch(table.childNodes[childElementCounter].tagName)
                    {
                        case "COLGROUP":
                            // Remove the COL elements from the DOM
                            var colGroup = table.childNodes[childElementCounter];
                            removeTableElementsDelegate(colGroup);
                            break;
                        case "THEAD":
                            // Remove the TH elements from the DOM
                            var tr = table.childNodes[childElementCounter].childNodes[0];
                            removeTableElementsDelegate(tr);
                            break;
                        default:
                            break;
                    }
                }
               
                // Finally remove the columns from the Infragistics CSOM
                grid.Bands[0].Columns = [];
            }
            else
            {
                // TODO: Handle this unexpected situation...
            }
        }

The above function resets a grid i.e. removes all rows and columns.

        /// <summary>
        /// Adds a new column to band 0 of an Infragistics grid using JavaScript.  This presents the opportunity of populating
        /// the grid without having to do a postback (i.e. the data could be obtained using much lighter-weight web services).
        ///
        /// NOTE: THIS CODE IS TIGHTLY BOUND TO THE INFRAGISTICS LIBRARY.  CHANGES TO THE INFRAGISTICS LIBRARY COULD RENDER
        /// THIS CODE INVALID.
        ///
        /// TODO: TEST THIS CODE WHEN UPGRADES ARE MADE TO THE INFRAGISTICS LIBRARY.
        ///
        /// </summary>
        /// <example>igtbl_addColumnToGrid('<%= TestGrid.ID %>', 0, 'Col1', 'Column 1', '150px', 'calculatorGridSelectedRowStyle', 'calculatorGridDisabledCellStyle', 'ig_b3617fe8_r1 ig_b3617fe8_r4 calculatorGridRowStyle');</example>
        /// <param name="gridClientID">The ClientID of the Infragistics grid</param>
        /// <param name="columnKey">The unqiue key of the column to be added</param>
        /// <param name="columnName">The caption of the column to be added</param>
        /// <param name="columnType">The type of the column as defined by Infragistics.  0 - NotSet, 3 - CheckBox, 5 - DropDownList, 7 - Button.  Simple textual values should be set to 0.</param>
        /// <param name="width">The width of the new column</param>
        /// <param name="selectedRowStyle">The selected row CSS class name</param>
        /// <param name="cellStyle">The cell CSS class name</param>
        /// <param name="rowStyle">The row CSS class name</param>
        function igtbl_addColumnToGrid(gridClientID, columnKey, columnName, columnType, width, selectedRowStyle, cellStyle, rowStyle)
        {
            var gridInputElement = $get(gridClientID);                          // Get the hidden Input element that surrounds the Grid object
           
            if(gridInputElement != null)
            {
                var gridInternalID = gridInputElement.attributes['name'].value;       // Get the Infragistics internal ID of the grid so that we can successfully add new columns
                var grid = igtbl_getGridById(gridClientID);                     // Get the Grid object from the Infragistics CSOM
                var tableDiv = grid.getDivElement();
                var band = grid.Bands[0];
                var indexPosition = band.Columns.length;
               
                // Create the column initiaization array which is used by the Infragistics CSOM to instantiate new Column object instances
                var columnInitializationArray = [columnKey,columnName,8,0,false,1,0,2,0,0,0,0,false,"",false,0,"","",selectedRowStyle,selectedRowStyle,columnType,"",[],"","","",0,[],cellStyle,"",width,true,false,false,rowStyle,"width:100%;height:100%;",false,rowStyle,0,"","","",false,"","","","",false,[],2,1,1,true,0,3,1,0,true,width];
               
                var table = tableDiv.childNodes[0];
                var childElements = table.childNodes.length;
                var childElementCounter = 0;
               
                // Step 1. Manipulate the DOM to add the new column
                for(childElementCounter; childElementCounter < childElements; childElementCounter ++)
                {
                    switch(table.childNodes[childElementCounter].tagName)
                    {
                        case "COLGROUP":
                            // Add a new COL element
                            var colGroup = table.childNodes[childElementCounter];
                            var col = document.createElement('col');
                            col.setAttribute('width', width);
                            colGroup.appendChild(col);
                            break;
                        case "THEAD":
                            // Add a new TH element
                            var tr = table.childNodes[childElementCounter].childNodes[0];
                           
                            var columnCaption = document.createElement('NOBR');
                            columnCaption.innerHTML = columnName;
                           
                            var column = document.createElement('TH');
                            column.setAttribute('id', gridInternalID + '_c_0_' + indexPosition);
                            column.setAttribute('columnNo', indexPosition);
                            column.setAttribute('height', '');
                            column.setAttribute('class', rowStyle);
                            column.appendChild(columnCaption);
                           
                            tr.appendChild(column);
                            break;
                        default:
                            break;
                    }
                }
               
                // Step 2. Add the new column to the Infragistics CSOM so that it's available for later manipulation
                var newColumn = new igtbl_Column(null, band, indexPosition, -1, columnInitializationArray);
               
                Array.add(band.Columns, newColumn);
                band.VisibleColumnsCount++;
               
                band.Columns[indexPosition].setWidth(300);
            }
            else
            {
                // TODO: Handle this unexpected situation...
            }
        }

The above function adds columns to grids.

The following code shows how the above can be used.

        function doAddTest()
        {
            igtbl_addColumnToGrid('<%= TestGrid.ClientID %>', 'Col2', 'Life', 0, '300px', 'calculatorGridSelectedRowStyle', 'calculatorGridDisabledCellStyle', 'ig_b3617fe8_r1 ig_b3617fe8_r4 calculatorGridRowStyle');
            igtbl_addColumnToGrid('<%= TestGrid.ClientID %>', 'Col3', 'ADB', 0, '300px', 'calculatorGridSelectedRowStyle', 'calculatorGridDisabledCellStyle', 'ig_b3617fe8_r1 ig_b3617fe8_r4 calculatorGridRowStyle');
            igtbl_addColumnToGrid('<%= TestGrid.ClientID %>', 'Col4', ' ', 3, '300px', 'calculatorGridSelectedRowStyle', 'calculatorGridDisabledCellStyle', 'ig_b3617fe8_r1 ig_b3617fe8_r4 calculatorGridRowStyle');
           
            var grid = igtbl_getGridById('<%= TestGrid.ClientID %>');
            grid.Bands[0].Columns[1].setAllowUpdate(1);
            grid.Bands[0].Columns[2].setAllowUpdate(1);
        }
       
        function doResetTest()
        {
            igtbl_resetGrid('<%= TestGrid.ClientID %>');
        }
       
        function doAddRowTest()
        {
            var grid = igtbl_getGridById('<%= TestGrid.ClientID %>');
           
            if(grid != null)
            {
                var row = igtbl_addNew(grid.Id, 0, false, false);
                var columns = grid.Bands[0].Columns.length;
                var columnCounter = 0;
               
                for(columnCounter; columnCounter < columns; columnCounter ++)
                {
                    var cell = row.getCell(columnCounter);
                    var column = grid.Bands[0].Columns[columnCounter];
               
                    if(column.ColumnType == 0)
                    {
                        // cell.setValue(columnCounter + " <b>Testing</b>");
                        cell.setValue(columnCounter);
                        cell.getElement().innerHTML = columnCounter + " <b>Testing</b>"
                    }
                }
            }
        }
       
        function getRowTest()
        {
            var grid = igtbl_getGridById('<%= TestGrid.ClientID %>');
           
            if(grid != null)
            {
                var rows = grid.Rows.length;
                var rowCounter = 0;
                var rowData = "";
               
                for(rowCounter; rowCounter < rows; rowCounter++)
                {
                    rowData += "\nrow " + rowCounter + "..."
                   
                    var row = grid.Rows.getRow(rowCounter);
                    var cells = row.getCellElements().length;
                    var cellCounter = 0;
                   
                    for(cellCounter; cellCounter <cells; cellCounter ++)
                    {
                        rowData += "," + row.getCell(cellCounter).getValue();
                    }
                }
               
                alert(rowData);
            }
        }

The function doAddTest() demostrates that once the columns have been added in this round-about-way, they can still can manipulated as if they had been added in the conventional manner by the Infragistics CSOM.

Hope this helps someone else!

NOTE: I have only tested this with the latest versions of IE (8) and Firefox (3.6.2) but it works perfectly in both.