With the right size all thread you could do it might be a booger to put together and I do not know if it would be strong enough but you have a good idea. The big bikes have a similar set up for people who have trouble with the heavy bikes and don't want to give up riding. Your thinking and that good keeps the devil from getting you to thinking of naughty things.
You said you have a bunch of front wheels laying around why not make an angle bracket and mount them on the back even if they are smaller there is something to think about you could bolt it together and not have to weld.
Norman

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norms-iron-works.tripod.com
motor assisted bicycle engine headlights that work off of your white power wire. check it out