If I had a 1500 wheel to try, I would first determine whether the 750's final drive would mate up to the 1500's wheel hub. If so, then that solves the drive-shaft issue and I could proceed to getting the rear disc brake adapted. The wheel hub internals look identical to the 750's; unless, the internal splines are different (different # of splines).
I feel the only difficulty would be spacing. I am uncertain if the 750's swing-arm is wide enough to handle the 1500's wheel and brake caliper bracket and spacer. If there is room, then the very least I would need is a new spacer made. Then I would need to fabricate a way for the bracket to remain in place (like our drum brake uses a bracer bar) by using the existing mounting bolt from the 1500. Once that is accomplished then figuring out how to adapt the rear brake rez and pedal to the 750 would be the final challenge.
I know this looks good in writing; however, I imagine it won't go as smooth in reality.
I only understand what I don't understand.