Base of the Rocky Mountains, Canada. Wonderous things!
2010 Cannondale Hooligan 3
I'm fairly sure that once its directly affecting the performance of the vehicle, it would be considered to be a part of the vehicle rather than an accessory. Ask a lawyer about if if you want a direct answer.
However, push trailers can get more than a bit touchy at the levels you're talking about. I would be more concerned about the sanity of such an idea rather than the legality.