The Duomatic turned up and it's beautiful. Gummed up from the feel of it, but I will strip it down anyway. It is plutonium-dense to pick up - only tiny but weighs almost 1.2kg. However, that's only about 250g more than a singlespeed hub, brake caliper and brake lever...and you do get an overdrive!
Any sources for FG 10,5 anti-rotation washers*? The 13/32" SA ones are so precisely made that they don't fit over the axle, and they look as if they would be a beast to file. FG 10,5 hub nuts are no problem - some are on the way from the Fatherland.
*all hub gears exert a small twisting force on the axle in all gears except direct drive, and the brake reaction arm doesn't help with this.