with vertical dropouts that virtually preclude use of a single-speed hub.
Not true , ,all you need is a spring loaded chain tensioner... its like A Rear derailleur that does not move in and out ..**
Universal Cycles -- Paul Melvin Chain Tensioner ..
Universal Cycles -- Rohloff Twin Pulley Chain Tensioners
there are single Pulley type too ..
Universal Cycles -- Surly Singleator ..
Universal Cycles -- 4-Jeri SS Chain Tensioner
Just needs be a freewheel single speed not a Fixed gear ..
** a short cage RD will do as well , just use the stroke limit screws to center it under your 1 cog..