PeterH:
That makes sense for a simple mechanical solution, but given the presence of the microcontroller and quite likely a stepper motor I wouldn't have thought that a reverse gearing mechanism would be required.
I am describing a purely mechanical solution which ensures that as he turns the spool at a variable speed, the sliding mechanism will stay in sync providing a uniform layer. He already has power from the crank which he mentioned he would leave to manual power anyway. Of course, if he is going to use the microcontroller, the reversing gear wouldn't be needed.