I have had several boats with feathering props with 2gm, 3gm and 3JH engines. In all cases putting the engine in reverse for a few seconds while moving at 5 kts or so feathered the props. I could then put the engine in neutral and the feathered prop didn't turn.

On a few boats with fixed props and these engines, I would put the engine in reverse and leave it there while sailing. The water flowing over the fixed prop produced a torque that often made it impossible to shift back to neutral before starting the engine. There were two solutions- disconnect the neutral start switch and start in reverse and then quickly shift into forward. The other solution was to head into the wind, slow down which reduces the turning torque and shift into neutral to start the engine.

I don't believe that leaving the gm or jh engines in reverse harms them any.

David