This sounds like a cool idea but could get rather complex if a planet changes hands multiple times.
I could also see an abuse potential if your little buddy civ is getting beat up. You give them planet back for points, then let the big bad civ take it again. You take it back, give it to your buddy for more points, etc. Tough to abuse on a large map, but could be easy to do on a small.
Would still like to see this...at some point.