I've never tried this while wheeling, but I often times use this same technique while launching a boat. Actually, this is how my father taught me to do it. Basically, keep the vehicle in a forward gear while backing down the ramp with a boat. That way, if the brakes go out, you can use the engine to stop the vehicle.
This should also work wheeling.
However, I drive a manual tranny, so I can't test it out for you..
Cliff