You may or may not have noticed when protecting XPS documents, that the usual list of RMS templates under the File>Permissions section is missing, even though you can see the templates in your other Office applications.
Just like the Office applications, the XPS viewer requires that you set a registry key in order to display the templates.
Here is that key:
Value: <path to templates>
BTW: This may only apply to the XPSViewer for Vista. I need to check.
Update: I checked this on XP, and you in fact can *only* use a Reg_Sz value, which means you can't use environment variables.