Hi Pete,

First off, let me apologize for the lack of a description on the sortMultiple method. The method is exposed as public due to implementation reasons - it's used by the multiple sorting's modal dialog if I'm not mistaken. This is why it doesn't accept any input parameters (because they're read internally).

As for applying the multiple sorting in a single API call, I would recommend doing so like this:

1. You need to access the igGrid's internal dataSource object and set the sorting expressions array for it. 
2. Call the igGridSorting widget's sortMultiple() API method.

(this will also update the grid's UI so no further steps are needed).

However, if you want to access the grid's data when it's bound to local (client-side) data and paging, sorting and filtering are all local (client-side), then I have the solution just for you.

It's basically the same as in the one from my reply on the forum thread you quoted but with one trick to it:

//Filter the grid with a good enough condition so that we can have at least one record on the 2nd page
$('#grid1').igGridFiltering("filter", [{fieldName: "Name",cond:"doesNotContain",expr:"Silverlight"}]);
$("#grid1").data("igGrid").dataSource.settings.sorting.expressions = [ { fieldName: "IsPostponed",  dir: "desc"},  { fieldName: "EndDate",  dir: "asc"}];
$('#grid1').igGridSorting("sortMultiple");


var filteredSortedDataSource = $.extend(true, {}, $("#grid1").data("igGrid").dataSource);
// we need to disable the paging feature so that the dataView() can represent only the filtered data.
filteredSortedDataSource.settings.paging.enabled = false;
// we need to make sure that sorting is applied to the filtered data
filteredSortedDataSource.settings.sorting.applyToAllData = false;

// Filter the copy of the igGrid's internal igDataSource in order to receive the desired data view
filteredSortedDataSource = filteredSortedDataSource.filter($("#grid1").data("igGrid").dataSource.settings.filtering.expressions);
filteredSortedDataSource = filteredSortedDataSource.sort($("#grid1").data("igGrid").dataSource.settings.sorting.expressions);

var filteredSortedDataWithoutPaging = filteredSortedDataSource.dataView();

The trick is in disabling the igDataSource sorting's applyToAllData option - then you can properly sort only the filtered data.
As usual I'm attaching a running HTML sample page where you can see my suggestion in effect.

If you have any other problems or questions, don't hesitate to let us know.

PS: Sorry about this requiring knowledge of the igDataSource, but when compared with the requirements we have for the igGrid and igDataSource, this isn't such a high priority scenario for us to provide an easy solution to.
By the way, there really aren't that many people (to the best of my knowledge, based on the forum and StackOverflow activity) that need to handle this scenario.
PPS: If you wish for us to devote some time and effort in order to provide an out-of-the-box solution, please feel free to post a new feature idea in the corresponding page under your account.

Cheers,
Bobby