I have had a casual look at the motor movers on the vans on site where I am and haven't found any that would impead the use of plastic ramps. I thought I'd do this because my van has an A frame mounted motormover ET which I only use for tight manouvers in storage, and because it's portable, I can store it in my shed.
I realy don't see why one would reverse up ramps as this put's two extra loads upon the clutch of the tow car. Firstly because reverse gear is higher ratio and often closer to 2nd gear, thus adding stress to the clutch pressure plate. Secondly there is additional force needed to overcome the brake's initial locking on, before the mechanisum releases.
Next is the safety aspects.
By reversing you realy need a banks person to guide you on site and to keep pedestrians away. The safest way is to, drive on and drive off, if it can be done. Also by driving forward up the ramps you can see clearly where you are going and only need the banksman to stop you at the required level on the ramp. Also by using 1st gear you will have the greatest control over your outfit.