The trick (or dirty work around if you like to call it so) is to generate a single dummy group using a invalid grouping field. The summary we require is specified as usual using the SummaryRow and SummaryField.
When this GroupingCollection is fed to ADG.dataProvider it would try to display in a tree view. By setting the HierarchicalCollectionView.showRoot flag to false to avoid displaying the single group parent.
By specifying the default ADGItemRenderer as ADG.groupItemRenderer we can prevent the group icons from getting displayed.The source is here.
Please note I have not tested this example extensively. So some bug fixes may be required before it can be used in production.