Why?

I don't see any reason why you need a chain idler/tensioner for a thru-axle? Tons of sliding dropouts that also use thru-axles. All City Super Professional was an off the peg option, Otso is pretty easy to get and Paragon Machine works makes at least one sliding thru-axle dropout and I know there are others that just don't come to mind at this time. Plus you have eccentric bottom brackets as well! There is really no reason why you couldn't run thru-axle as a single speed or fixed gear aside from maybe an issue with a full suspension bike but that is because of the full suspension and I wouldn't really want a fixed gear full suspension and loads of options to tension the chain for single speed including a derailleur.