This is a very good idea for a secondary game mode. I've been considering it a lot, and the original idea SB has sounds fine considering the following facts:
-You can already drain enemy AP by throwing them
-You can block supers with (a) certain weapon(s)
-You can hault supers from being performed with (a) certain weapon(s)
-You can always dodge supers
Provided that these work consistently, it should be fine. Remember, the higher supers are meant to be easier, though I do share concerns about balance. Why choose Radec is his supers are harder to do than Sweet Tooth's
I also think the idea of having your AP explode all over the map after your special rewards the enemy players for no reason at all. The enemy has enough opportunities to gain their own AP. Your suggestion punishes players for using their 3rd super.
I really do like your idea, as a sort of tug of war mode. I feel that doing damage should build AP, but also that you should be able to drain enemy AP by landing perfect combos/enough blows. This way, not everybody would build to level 3 and refuse to use lower level supers in fear of losing AP. You should generally be able to build AP faster than you lose it, but if you generally get attacked more than you attack others, you should be losing AP