I have a Rotator SWB that I bought used. It has a mid drive, with a 5 speed mid and 7 speed rear cluster (mid is fixed, rear is a freewheel). It uses twist shifters, the front has a 7 speed but only uses 5 of them.
I think it is great. The mid shifts much better than the front on a traditional setup, and you can shift either one depending on where you are on the respective clusters, until you get to the highest or lowest gears.
I can see why this would not work on a DF, but wonder why it is not more popular on recumbents.