Hmmm...good question...let me think. It would cause a problem with AWD vehicle as you surmised.
A regular vehicle (RWD) would be simple to do since as you drive onto it you are pushing the front end on without varying the speed of the drive wheels. As soon as you hit the ramp you could just let off the clutch and coast up the ramp (if you had enough acceleration in relation to the moving ramp) then shift into first and pull up[ the ramp as if starting on a hill. Or you could just hit the brakes without hardly a lurch as you will be shedding off almost no speed except for the rotational force of the wheels themselves.
A FWD vehicle would be a bit trickier to do smoothly and would take a skillfull driver to pull off nicely. You'd have to hit the ramp with a little speed in relation to the ramp and let off the clutch just as you hit it, letting the momentum carry you up the ramp long enough for you to shift into first, allow the revs to bleed off, then simply drive up the ramp in first as if you were starting on a hill.
I think it could be done nicely on a RWD or FWD vehicle. An AWD would have it's center clutch reeked havoc upon due to the rotational speed differences with the axles.
The thing to remember is that the speed differential is not really all as great as your subconcious tells you it is. It's not like doing 60mph then having to stop instanly or shoot off the front of the car carrier. The car and moving ramp are moving at nearly the same speed so there is no real shock of speed differential when you hit the ramp. The only thing that has to stop quickly is the rotational momentum of the wheels and since they weigh considerably less than the overall mass of the car itself, you would barely feel it's effects.
Sure would love to get a chance to try it out one day...With someone else's car of course.
-Johnny (Fast and Irritable)