I guess I can understand your logic, but locking down a shaft would still be the simplest way to go. I have a fair amount of single engine running with a twin engine boat experience, and the easiest method I found was to take an open end wrench and place it on one of the bolts at the tailshaft of the transmission in such a way that it locks against a stringer. When you find yourself in need of that particular engine/transmission, just fire it up, throw it into reverse, and the wrench comes off without ever having to leave the helm. That comes in real handy in an emergency situation.

Granted I never tried that at any speed to speak of, so I'm not sure what you could get into with loosening or overtightening a bolt like that.