You're definitely on the right track - the sprocket and flange I was using was freewheeling. I had the idea to do removable pins in the bearing casing, but ended up just JB Welding the bearings to frozen. Of course, the reversed threads on the motor side of the hub is a different dynamic than what you're doing, but the pins is a good way to go about it, it should give the proper reinforcement to hold strong to the high torque low rpm side of these motors.
Best thing I could ever think was to weld a flange to the hub and bolt on and off the sprocket directly from the hub. A flip flop seems like it'd be a good candidate for this, but I haven't ever welded anything more than printed circuit board and wires (soldering
). Welding the lock nut and sprocket/flange to the hub is a good way to get things started. It's in essence the same concept as the disc brake hub, but without having to go purchase an all new back end.