Originally Posted by
Sixty Fiver
With vertical drop outs you could use a road derailleur as a tensioner and even play with setting up a dual drive on an SA hub to expand the gear range.
Even if the hub you choose can only take a single sprocket, there's the option of achieving the same thing with a front derailleur and a double or triple crankset. You could combine a wide-range hub with carefully selected chainrings to give half-step gearing, or maybe experiment with third-step on a triple crankset