A quick update, I had trouble getting the internally threaded bushing to work, too long to explain here ...
However I have managed to get a working solution by brazing a Capreo 10T cog to a standard Shimano steel lockring. This requires a little machining off the back of the 10T cog to get the spacing correct, and then turning down the diameter of the lockring collar after brazing, so the chain does not foul the lockring.