I do not think anyone has done this before, but why not make a game of paintball on all three floors of Sandbox. Throw in some randomly placed cover and make sure the gametype is on instant respawn, and it could be really fun with 16 people playing.
I personally think it would be more fun on one level, Halo isn't meant for massive combat, meaning 16 people would get spread out too much in my opinion. Besides, how often do you get a party of 16, let alone 16 who have sandbox?