I figured Mario was the safe choice for those who feel I shouldn't be that risque or that color coordinating is more important than what the shirt says.
"Punching trees gives me wood" is certainly obviously naughty, but the creeper one has its own innuendo. A somewhat phallic looking creature that chases after you, and when it gets close enough it gets excited and explodes? :) And of course there are potentially other advantages to wearing a shirt that encourages people to give you hugs ;)
On the other hand according to the forums the event has gotten popular enough that they get more volunteers than they need, so they're filtering based on who they think will make good pin-up material =P I'm not sure which option would best improve my meager chances of making the cut.