Expanding child records above, rather than below, the parent in a TreeView or grouped XamDataGrid

Question

I have an Infragistics XamDataGrid showing records grouped by department. When I click on an expansion indicator next to a department name, the grid shows me all records with that department as a subgrid BELOW the group name. I would like to display the subgrid ABOVE the group name.

I realize this may involve some pretty serious re-templating so if anyone has a solution involving a TreeView I'd be interested to see that too.