It is supposedly not designed to do that -- the clutch and rest of the transmission aren't set properly.

I wouldn't put it reverse with the engine running and boat going forward -- over time, I would suspect that would cause more damage.

The increase of forces isn't proportional on a linear basis -- I think it would a fraction of either difference in speed squared or cubed. So...if you are going to be sailing at 8 knots vs. 4 knots, then the torque would be signifiantly greater.

Having said this, the tech desk at Mack Boring offered that "in the old days", everyone used the reverse for standard marine transmissions, the newer (improved) models don't like this over time.

I'm just reporting the news -- I don't have any further insights. (If it were me, and I didn't have a feathering or folding prop, then I'd probably use reverse to stop the free wheeling.) But I have a two-blade Max Prop...