Many transmissions won't allow the gears to back up. If you are rolling backwards and through the thing in forward on many trannies the thing locks up. The one way clutch just stops it BAM!! Hondas are like this. I don't think any MBs do this.
If you are on a big enough hill and the engine speed is low enough the tranny will back-up. It is easier to see by doing the manuver above; the placement in drive while still moving backwards.
One thing to remember is that if you go in reverse while in drive the relative speed (one fan versus the other)goes up quickly do to the high (numerically)gear ratio of reverse.
I would say that there would be no more load in doing this than reving the motor (while in gear) to say 1000rpm while holding one's foot on the brake.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician