No clue how to make maps for that. And how would you play invasion on Custom Edition?

I bet it could be adapted into a campaign level at least.

It would be a mess. It's difficult to synch AI between a client and a host in CE, and even more difficult for the AI to synch between clients. I've seen some guys come close, but it was never perfect. If you're talking about having real players with dynamic levels, that would be possible, but dynamic environments have a lot of the same problems with synching. I'm talking half the people in the room wouldn't be able to move past a stage-one barrier.