Alright, upgrading to disc brakes. Hope make some lovely hubs, the RS4 are centrelock, so I'll buy some TRP centerlock rotors to go with them.
Neither wheels nor rotors come. Fair enough, they're not expensive so I'll buy some token 35 mm lockrings, and the appropriate tool for 12 mm thru-axles.
I've fit them and there's a good half-millimetre of open air between lockring and rotor.
By best measurement, the lockring is 35 mm outer diameter and 4.5 mm deep. The rotor is 11 mm deep. Results are identical with a sram lockring.
What gives? I think it might require a different lockring, but there's no information from any of the suppliers involved, or even a decent selection of lockrings in the first place. The only things that might fit the rotor (41.7 mm outer diameter?) appear to be for 15 mm axles only.