The only thing I can think of is to go with a similar set up to a wiper motor, where the drive bar actually sits on a conical spline, and the nut just holds it in place. If it is actually a wiper motor could you use the drive bar from the wiper system bolted to the back of your wheel, then bore out the centre of the wheel for access to get the nut on. This would certainly handle the torque. Might be a case of buying a wiper motor and drive from a scrapyard to source matching parts.