Hi Bryan, 

This is an interesting puzzle. The dialog itself is not made to be modifiable. There's no list of items you can add to.

But I don't think it would be too terribly difficult to write code that modifies the dialog and adds an additional Checkbox.

I took a brief shot at it, and it seems pretty straightforward. Here's the relevant code.

        private void UltraGrid1_BeforeSummaryDialog(object sender, BeforeSummaryDialogEventArgs e)
        {
            // Find the controls we need on the existing dialog. 
            Control summaryControl = e.SummaryDialog.Controls["SummaryControl"];
            Control backgroundPanel = summaryControl.Controls["ultraBackgroundPanel"];
            Control cancelUltraButton = backgroundPanel.Controls["cancelUltraButton"];
            Control okUltraButton = backgroundPanel.Controls["okUltraButton"];
            Control panelCheckboxes = backgroundPanel.Controls["panelCheckboxes"];
            Control sumCheckBox = panelCheckboxes.Controls["sumCheckBox"];

            // I am assuming that the application allows multiple summaries, so 
            // the dialog is using CheckBoxes. For a single summary, it would use radio buttons
            // and you would need to use these two controls and change the code below to add
            // a RadioButton insetad of a CheckBox. 
            //
            //Control panelRadios = backgroundPanel.Controls["panelRadios"];
            //Control sumRadio = panelRadios.Controls["sumRadio"];
                        
            this.myCheckBox = new CheckBox();
            this.myCheckBox.Name = "myCheckBox";
            this.myCheckBox.Text = "My custom summary type";
            this.myCheckBox.SetBounds(
                sumCheckBox.Left,
                sumCheckBox.Bottom + 4,
                sumCheckBox.Width,
                sumCheckBox.Height
                );

            if (DoesCustomSummaryAlreadyExist(e.Column))
                this.myCheckBox.Checked = true;

            // You will proabably want to add an image to the CheckBox here so it will look 
            // consistent with the other checkboxes. 
            //this.myCheckBox.Image = myImage;


            // The dialog and the controls need to be shifted and resized to accomodate
            // the new control. 
            int heightAdjustment = this.myCheckBox.Height + 4;
            panelCheckboxes.Height += heightAdjustment;
            e.SummaryDialog.Height += heightAdjustment;
            okUltraButton.Top += heightAdjustment;
            cancelUltraButton.Top += heightAdjustment;

            panelCheckboxes.Controls.Add(myCheckBox);
        }

        private void UltraGrid1_AfterSummaryDialog(object sender, AfterSummaryDialogEventArgs e)
        { 
            if (null != this.myCheckBox)
            {
                string myCustomSummaryKey = GetMyCustomSummaryKey(e.Column);
                bool customSummaryAlreadyExists = DoesCustomSummaryAlreadyExist(e.Column);
                if (this.myCheckBox.Checked && !customSummaryAlreadyExists)
                {
                    // I used a Formula summary here as a placeholder so I didn't have to 
                    // create an ICustomSummaryCalculator. But you would use SummaryType.Custom
                    // and use an overload of Summaries.Add that takes an ICustomSummaryCalculator.
                    e.Column.Band.Summaries.Add(myCustomSummaryKey, SummaryType.Formula, e.Column);
                }
                else if (!this.myCheckBox.Checked && customSummaryAlreadyExists)
                {
                    var ss = e.Column.Band.Summaries[myCustomSummaryKey];
                    e.Column.Band.Summaries.Remove(ss);
                }

                this.myCheckBox.Dispose();
                this.myCheckBox = null;
            }
        }

        private bool DoesCustomSummaryAlreadyExist(UltraGridColumn column)
        {
            string myCustomSummaryKey = GetMyCustomSummaryKey(column);
            return column.Band.Summaries.Exists(myCustomSummaryKey);
        }

        private string GetMyCustomSummaryKey(UltraGridColumn column)
        {
            return string.Format("MyCustomSummaryColumn - {0}", column.Key);
        }

And I also attached a working sample.WinGrid - Custom Item on Summaryt Dialog.zip