[Cuog]

A less sophisticated version would be to use two attached wires similar to speaker wire in the appropriate gauge. Run it down through the center of rotation, you could then experimentally determine how much slack you'd need in the wire and how many twists you could handle before damaging the wire.

I know when I build tube amps I do something like this with the AC heaters and I'm able to get a good 10-20 tight twists in a 6 inch length. Using a little bit of clever coding you can ensure that your robot wont exceed so many twists in one direction.

Certainly not as elegant as a slip ring, but quite effective within the rules as I understand them.