In the Q1 RadControls for Silverlight you can directly edit all control templates in Blend. You simple open the control in Blend, create a copy of its template, and then you can begin directly editing the XAML. The changes will be stored directly in your project, so you can easily edit them at any time. This approah is available for editing the default RadControl templates; for editing the non-default themes a little more work is required (similar the approach you've used in the past with generic.xaml). Give Blend template editing a try and see if that works for you.
Hope that helps!