I think the pull/linkage ratio will still work, but since the newer chain is narrower, the pulleys are thinner, too, and so the space between the cage plates is less. Don't know if it's enough to rub/seize the wider chain or not, but it might be.

I do know when I cannibalized a 9-speed Campy RD to renovate my 8-spd, the pulley bolts on the 9 were actually shorter, so I'd assume it's sandwiched even tighter with a 10-spd version.

Yea I was thinking an 8 chain might run on the cage. It's only a single speed so no FD issues, but I will probably just grab some 8 speed cranks, chain and RD and be done with it. Thanks