The event args from column.groupHeaderTemplate (and groupFooterTemplate) should contain a function to retrieve the data for that group. Currently, they only contain "field" and "value." If you made the grouped rows available in this event, clients would be free to add multiple aggregate columns of the same type (i.e., get the "sum" of multiple values instead of being limited to a single value). I've been able to halfway implement this, but it only works correctly if I'm only grouping by one column. For example, if I'm grouped by WidgetName and then by ProductCode, my event args for ProductCode should contain a function to return data rows for only rows matching the current ProductCode and WidgetName.