I understand what spinning magnets is saying. But correct me if I am wrong if a rider wants access to lots of "power", higher voltage and low amps are a much simpler or could I say logical choice. It's the high amp side of controllers which gets bigger and less attractive. How the RC / astro guys go with those tiny little controllers is another field I am scared to venture into.... The complexity is the scary thing for me at least. Only thing I like about low volts is cheap chargers.
Back to mechanics, complexity is not just a concept that you deal with once and for all. It is opening up a multitude of things that if can go wrong eventually will.
The only thing that has not gone wrong in some way on my LR big block and bicycle drive train connected to it is the motor itself and the bracket that holds it to the BB.
That's a fail every few rides and it has never been powered up over 50 degrees celcius! Meaning it's got so much potential to unleash for those with a labor of love.
Back to your placement, if running through the BB and the only place you can install your motor is up high, again I think you haven't really created a better set up than it's "normal" in front of the bb spot. As you know as low as possible is best. Only advantage might be less front wheel dirt to deal with.
If you have access to someone who can customise the swingarm more specifically for the big block though, I think you might be onto something special with a direct say #219 14t to 90t.
The motor width to deal with might be overcome with something like a fat bike rear hub and bottom bracket? Or some crazy bicycle chain jack shaft to send pedal power up and or around the motor area. Would rather have bicycle chain which will barely ever used up high just for legality than a motor up high.....
enough rambling, but this is the area I am very interested in finding solutions to myself. have fun