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There's a CXC on radavist with 6 bolt cog on the disc mount and problemsolvers 6 bolt adapter for the disc brake on the freehub body.
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So, is he using one of these, with a disk rotor in it for the rear brake?
https://problemsolversbike.com/products/cog-carrier
Brake doesnt work rolling backwards i guess, which is of little concern but will still be odd. Must put a fair bit of force through the freehub when you grab it though.
Checkpoint
But seriously if you're going to build a fixed road frame would you going for 120mm or thru-axle/disc on the rear? Something like this with the sliding dropouts, then stick some carbon thru-axle disc wheels on it which you probably already have spare