A festival of close clearances
I mocked up Silas’s Shiromoto with the only 26" wheels I had in the house – the wheelset from the ten-ton-trek, which has 1.6" road semi-slicks on it right now – and discovered that the clearances are close enough so that 559×50 wheel sets are, not surprisingly, too big (if 650×32b doesn’t fit, something that’s 13mm larger diameter won’t fit either.)
So I need to stick with 559×25 (Ritchey Tom Slicks, or similar, which will give me barely enough room for tire + fender) or consider dropping down to one of the three 24" sizes (whichever one has tires that do not suck; there are a bunch of bmx tires out there, but, alas, no bmx-dimensioned Resist Nomads :-( ) instead.