I have been boating over 50 years and never lost a transom on the highway. Actually never lost a transom to any source. I guarantee you that the shock load the transom gets from the engine, which is usually positioned somewhat up to keep the skeg clear of the roadway and thereby balances out some of the dead weight, is MUCH less than the shock you get at WOT bouncing across rough water. My last boat was bought used and had one which I chunked it. Course my engine had PTT and on non PTT engines I did block the engine up as the tilt bracket wasn't designed for trailering and I could see the engine in my rear view mirror, which I checked regularly as I checked other things while driving.
I did have an 18' Starcraft I/O once that was a restoration job. It didn't have PTT and being an I/O had nothing to counter balance the outdrive. Last I couldn't see it's position in my rear view mirror. On that I did use a rod that was designed by Mercruiser for trailering and it attached to the top of the outdrive gearbox on one end and had fingers that hooked over the pins where the tilt mounted to the mid section on the other.
However with all that bla bla, neat idea. Thanks for sharing.
Mark