Fixed/free hub for 12mm TA frame?
Flipflop disc brake hubs don't exist. If you were willing to ride without a rear brake, you could switch the existing wheel back and fourth by using a bolt-on track cog. I believe they give a chainline in the 41-42mm range. This plan would however be asking for trouble if you were using a hydraulic brake, since it wouldn't have its rotor in place to prevent the pistons getting pushed out.

135x12 is weird enough that I doubt you're going to find a fixed gear one stock, but supposedly Paul Components has made 135x12 singlespeed freewheel hubs, and they're the kind of company that you might be able to convince to make you one for a track cog, since the shell would probably be the same with the addition of lockring threads (unless it was too thin, in which case nevermind.)