I'm giving this a nudge over to the Transportation forum, since I think that's a better fit for questions about preparing/building a vehicle. The MV board in the 2012 event board is more about on-playa activities.
When my girlfriend and I did some transformation work on our van so it could function as both an on-playa mutant vehicle yet still remain street legal, we didn't put any special coverage on it. In our case though, it's only a mutant vehicle when quite a bit of extra structure is added (we like to think of it as a playa costume), otherwise it's just a cargo van with a custom steel roof rack. And for us, the 'art car' mode is pretty much a chill space (as opposed to parade float/mobile dancing platform), so there isn't any real concern about a collision with a bunch of passengers on board. Depending on the kind of vehicle or number of passengers you plan on carrying, you may want to increase your liability coverage.
Any MV builders or operators out there that care to weigh in on this?