I have been shopping for a frame ever since I started this thread. I still haven't found one yet. I have been focussing on 29er frames because they seem to have the best combination of features to do what I want. They almost universally use disk brakes, so I may have to go that route, instead of the internal hub brakes I want to use. One wierd part of the 29er world is frames are sold without a fork. So, even if I find a frame, I still need to find a fork. In the end, I may end up buying a complete bike and just swap out the rear hub.
P.S. - I even found a new bike, a complete bike, with the infinitely variable rear hub for as little as $400. I haven't found any inexpensive Rolhoff hubbed bikes. :-)