Hi Ben,
On a couple of occasions I have, added a tap to a regular control. In both cases I used a 4 watt clarostat wire wound control. The procedure is a bit of a pain but it can be done.
First remove the wiper and control shaft from the body. Then I grind off the 2 rivets that hold the wire element and end terminals into the bakelite body. Now you can remove the entire resistance element. Then using a Dremel I make a slot in the bakelite body where the new center tap terminal is to be located.. I make a new terminal of brass shim stock about 1/4" wide and 1" long and slide it through the slot I made in the bakelite body. Then I re-install the wire wound element into the body in such a way that the brass shim stock is wedged tightly between the outside of the wirewound element and the inside wall of the bakelite body, thus making a tight mechanical/eelectrical connection. Then re-assemble the original terminals at either end of the wire wound element replacing the old rivets with tiny brass screws, nuts and washers. Re-install thee wiper and shaft, and you're in business.
You will need a pot like this one to do the conversion.
http://www.ebay.com/itm/CLAROSTAT-50...3D232072262896