I take your points about using branches, however:

If you spin-off a part of the development effort into a version-control system that is entirely disjoint from the one that the enterprise as a whole is using

That's not what I've done. Essentially, I have a "scratch" area where I can try things out, because the corp. policy says "no check-ins of non-working code". When I do fix a problem or add a feature, that immediately gets synced and checked into the corp. CVS -- so it isn't disjoint.

I essentially did with CVS what git seems designed to do (and better) -- created a local version history for code that isn't ready to share with others yet.

Based on your comment and others', it seems like the only real option I have is to try to get policy changed to allow me to create "unofficial" branches that contain non-working code, or to continue to do the bastardized process I have today.