I haven't tried this.....but couldn't you just put an unbound dropdown on the screen showing all the method report templates for invoices (ReportMethod table), and then in the Generate Report action set "Override with Report(optional):" to Value From Screen, the dropdown.
BUT - are you talking about Method templates, or QuickBooks templates? If you are talking about the QuickBooks templates, just drop the "Template" field on the screen.
Paul