OK, wee update, finally got around to having a play with the 9/10 speed setup, using a (non-clutch) XTR rear and an old X9 shifter. Stuck with the 9sp chain to match the cassette.
First obvious thing- it does kinda sorta work as standard. I can see why people think it does work- it’s no worse than a lot of people’s shifting is normally. If you set it up so that it shifts perfectly at one end it’s far enough out to not really work at the other, but if you set it up to work perfectly in the middle then the ends are flaky but not outright useless. Sort of end-of-the-muddy-ride.
So, on with the 1/4 inch of spacer. And it’s definitely better. I didn’t get it running perfect, partly because the cable kept pulling out of my bodgy spacer and I got annoyed. I’m not quite convinced the 1/4 is right, I need to spend a bit more time actually watching it go. But I’m reasonably sure that it’ll work, now.
Oh yeah and the 9-speed chain seems perfectly happy in a 10-speed mech. I might see if I have a suitable length 10-speed chain to try on the 9-speed cassette.