What make of mixte? I just picked up a nice, mid-80s lugged Nishiki
Century (mixte) for my daughter at college. It came with 700c wheels
and riser bars, and for her use I think I'm going to leave it as-is
for now. But eventually I'd like to repaint it and lace up a set of
650B wheels. This would require long-reach brakes or drums... hmmm...
Are you using drum brakes for the rear wheel too? (I've laced up a
few sets with the SA 8-speed drum hubs... like em!!!!) I think
there's also a freewheel or cassette version made by SA with drum
brakes, although I can't recall.
I also have build a few bikes with the SA drum-dynohubs. You can't
beat em for the price; they work great.
Please post pics when you can. GOOD LUCK!!