Any other less-expensive BFs (I'm thinking Pocket Sport, Pocket 8) with horizontal dropouts?
those use a derailleur.. realistically there is a fold/hinge point behind the BB,
so You should just consider, instead of a fixie, 2nd best in simplicity,
a single speed (or a 3 speed IG hub) with a chain tensioner,
as the chain goes slack as the rear wheel is folded forward.
same basic theme with the Llama, the chain tensioner and hub on the one I just got was from Rohloff.