[Al Skierkiewicz]

Why not have a wheel encoder tied to a non-driving wheel. Use two at ninety degrees to each other using omni wheels and then you don't have to worry about running wires down inside the modules.
Please keep in mind that bevel gears (those normally used on coaxial drives) are relatively inefficient in use for driving and require some pretty meaty bearings to keep them aligned under load and stress.
Slip rings can be made by using circular, machined conductive surfaces, insulated from the shaft by PVC, polyeurathane or delrin. Brushes can be bought anywhere, even at Ace or TruValue. All you need to do is keep them aligned in the assembly and have some form of attachment. Using brass allows you to solder wires directly to the rings.