So what you're suggesting is that, essentially, the front wheels of the Prius would be on a ramp on the back of the bus, with the steering unlocked, and the rear wheels would be on a little dolly to keep the prius rear wheels from turning? (Basically, the Prius IS the trailer frame)?
It's a neat idea, but I'm not sure if it's technically legal. It would also do some really interesting things to the suspension setup, because it would put half the cars weight on the rear bumper of the bus. (Then again, tow trucks don't routinely tip over, and they've got a similar weight distribution at times.)