Very nice idea fliping the bicycle upside down. and the piviot point from the stock headset. Simple advice too make it cheaper... use a old rear-end(seat stays, chain stays, seat tube and drop outs) form a donor bike. Im going to make one of these for a friend too pull around. for free!
Aight so... Sorry to bomb your post. Ive built few electric push trailers. Same idea you have going on hear, but with old kid scooters(electric, gas or foot push type) But battery's go dead and are heavy. so i have plans on a gas push trailer. I already have the old razor 47cc stand-up scooter. Murder it and bolt a old cut mtb rear end infront of the pivot point. some type of linkage for carb butterfly and a kill switch. Bolt on any bike your heart desires that day.