I go round and round in my head whether this is a good idea or not. When I was growing up, we had motorcycle size classes within our endorsement based on the size motorcycle with which you completed the practical. The written was the same but the classes broke down to under 150cc, under 500cc and unlimited. The reasoning was that someone would pass the test on a scooter and then hop on a larger bike that they could not control. Although I see some benefit to this method with first time drivers, I believe even experienced scooter drivers can apply what they know and go to larger bikes.
Perhaps a beginner's permit with restrictions for the first two years would be a better option. Until you have no violations or accidents during that length time (a clean 2-year period), you do not get an unrestricted license. If you start racking up points on your license (because you cannot handle this new class of vehicle), you have the choice to pay for formal training (any violations in the next two years would revoke your endorsement) or give up the privilege of this endorsement.
I carry endorsements on my license for unlimited motorcycle displacement with additional sidecar or trike operation. We do not have an endorsement for RV's over a set weight or length, although I do feel we should have towing endorsement that would cover anything you hitch to your vehicle (FW, BP or flat tow).