Hire a sag bus. And, just to taunt them, charge a rate for every mile they don't do.
(And the sag bus might actually be a lot of fun)
While I agree that the people in the SAG vehicle will probably be having a LOT more fun than those NOT in the SAG vehicle, I think it misses a fundamental issue:
First: Why would anyone do something so crazy just because somebody challenged them to do it? It sounds like the scene from 'A Christmas Story' where they "Double Dare' the kid to lick a metal lamp post on a freezing cold morning...
Second: Why would they agree to pay a fine for every mile they don't complete?
Who is the winner in this deal? All I see are losers...