ok...if you do spin the motor backwards you're saying metal to metal contact, no lube (from the fuel) will cause scratches.
well, if you put a few drops of after run oil in the cylinder...you have lube and you can spin it backwards.
bo, you can not compair a nitro engine to a car engine, or even a motorcycle 2 stroke for that matter.
motorcycle 2 strokes have either reed valves or they have rotary valves.
as far as i know, a nitro engine does not have any moving valves...maybe some slots for intake & exhaust, but no actual valves.

in contrary, you can run a diesel engine backwards...i have seen it many times.
you can also run a 4 stroke car type engine in reverse (all in-board car type boat motors run in reverse.)
you have to make changes to the ignition to make one run backwards, and you need a reverse flow waterpump, and a reverse rotating starter.