Do they make dry-erase board paint? maybe a clear wrap so the whole truck becomes a big dry erase board. like was said before, let kids at shows go nuts drawing whatever they want, then enter it for best paint job.
I say do it. But be prepared for somebody to use a paint pen and draw stuff that can't wipe off. Might be a lame one, like a dick. Or it could be super awesome like a mural of xhibit saying "you boy I heard you got a truck, so I put a toaster in yo glove box!"
And everybody is liable to end up with stuff drawn on their rides around me. I carry a colorful assortment of dry erase pens for people's windows. And every car has those.