There are hubs with 6-bolt disc attachment both sides, so you could fit a disc and a 6-bolt sprocket. They come in 20×110, which is easy to adapt to pretty much any axle standard, although you'd need to make new end caps as well as axle/tube/spacers for 100mm OLN front types.
Do rear fixed wheels with disc tabs exist?
Considering a new build and would probably go disc brakes.
(can be centre lock or 6 bolt)
(May not fit a rear brake, but want matching wheels because reasons)