Found a disc brake conversion kit on Amazon last week. My daughter was ordering some things from the "Zon" and I had her add this to the order. It is made for vertical drop out and aluminum frames, however I do believe I can do a few mods to make it fit this 1986 Fuji Sundance frame without too much fuss.
Right now in mock up the wheel spins just fine. Next step are the necessary mods to secure it to the frame and clear the fender and rear rack. Once done it will function with it's own lever as a drag brake much like a tandem has a drum drag on it.