Why? I think it's a brilliant idea. The only problem I see really is that it doesn't take enough advantage of the hide-and-seek blocks in the center for flag-carriers, and that could be fixed by moving the openings to the flag spawns away from each other (perhaps not the totally opposite side, but maybe through a tunnel directly underneath, or even both). Only-Player-Collides polygons could be really cleverly used to allow players to take another stab at winning (allow each flag zone to be shot at from the other side that can't be entered).
You should pursue this.
