Not what I had in mind but certainly gives me some ideas....There are similar designs all ready out there.
Just do a y a h o o search on the idea.
Here's one for starters:
Osmos wheel- designer and manufacturer of hubless orbital wheel Geneva GlobeHolding SA
Yeah, there are no spokes or a hub, but a rim drive is just one way to transfer power.
That was one of my crazy thoughts that I nixed as to ambitious/complicated. Use the flex shaft from a weed wacker to power some sort of roller head on a caliper brake type set up, if you put an idler roller on the other side it could squueze the rim quite nicely and if you are just moving the roller it could easily be engaged/disengaged with a brake lever type system.If I came across a light weight vertical shaft engine I might try to work it out like a brake on the rim