Shameless plug:
Unofficial Xcode 4 Template Documentation
I've put in roughly 70 hours to document the Xcode 4 template system as good as is possible with trial and error. It's 70 pages plus several example templates. I charge a small fee for it because it's extensive and I put a lot of time into it.
I was most interested in figuring a way to add other Xcode projects to a project template, that actually got the documentation effort started.
Unfortunately, all my tests did point to one conclusion: Apple never intended for project templates to use or create additional project references. You can add a .xcodeproj file to the project template, but the new project template format unfortunately doesn't allow you to copy files without adding them to the newly created project. In essence I would say that creating project templates referencing other Xcode projects may be possible but not in a satisfactory way (speak: no manual cleanup by user of the newly created project). At least not to me.