32T is the smallest for 104 BCD

Not square-taper, but SRAM is supposed to offer an integral-ring 26T crank in the XX1

Cyclotouriste crank arms are 50.4mm bcd, but the smallest "outer ring" TA makes is 40T.

The 74 bcd ring stack on a Cyclotouriste can go down to 24T.
Probably not cost-effective, but there might be a way to turn a TA outer ring into a chain guard and mount a small ring to the inner position.

wait a minute - try these guys Cranksets - New and Vintage ? Bespoke Chainrings
They should be able to make you a 50.4mm bcd chainring in whatever you want (down to 24T)
Cranksets - New and Vintage ? Bespoke Chainrings