I acquired a singlespeed disc brake bike because I assumed it would have a reverse thread lockring under the freewheel. It did not.
I found the disc spacing is the same as the MTB singlespeed chainline of 52mm. There are adapters on Amazon to thread a six bolt disc brake mount on the freewheel threads. There are also sprockets which have six disc brake style bolt holes.
So the plan is to turn the wheel around and put an adapter and disc brake on the freewheel threads, and bolt a fixed sprocket to the disc brake holes. Since the threads face the other way, the braking force would tend to tighten the disc.
The question is, is it strong enough? I am not a strong cyclist any more but am a clyde so the brake at least would be under considerable force.