There is no registration at defcon, it's all anonymous. You hand them cash for a badge and a packet of info/stickers/cd and are on your way.
(I suspect the main objections to indoor paintball will come from the venue. Perhaps a less messy, but still humiliating, method can be used?)
That said, that's the least important reason, the more important one being that you can't just nail people with any kind of gun at an infosec conference, no matter how unconf it is.