That wouldn't work either, Karnak. GV's don't have even a fraction of the aircraft's ability to go land at another base if theirs is captured. If you remove their ability to land anywhere as long as theres no enemys within 6k, you're being a bit unfair to the GV'ers.
The ground war wasn't a war of bases and strong points like AH depicts it. As long as troops weren't behind enemy lines, and they were still operating as part of a cohesive unit, they would still recieve supplys. Do you think that Panzer units stopped getting gasoline shipped to them as soon as they got pushed out of a town?
That being so, GV's shouldn't require a base to land safely at all. They're nessecary for gameplay purposes, but we shouldn't by trying to further restrict GV's.
Personally, I think a delay between when you hit the end sortie button and when you actually land would be best. To keep it fair (to say nothing of realistic), it should also be applyed to aircraft as well; your plane stayed on the runway even after you hopped out of the cocpit.