Making trailers may be a red tape nightmare, but using a recognised manufactured form of a bike frame may help navigate the maze.
Been looking at Singlewheel.com again, and wondering if this is a go-er, so talking to engineers.
Perhaps 2 frames could be joined in tandem?
Obvious option is to put trailer ball where fork yolks would have been. Either extended or below.
Singlewheel - we've compiled a list off all the manufacturers of Single Wheel, 1 Wheel, Mono, Uni Wheel trailers http://www.singlewheel.com