Well, I tried it, and it worked great. I had a system error on the first boot, but rebooting without extensions and manually moving a bunch of junk from Extensions to Disabled Extensions solved the problem.

FWIW this is a used Mac with a lot of older software on it. Next thing will be a fresh install of 7.5.5 and getting Ethernet working, but I'm sure I'll manage.

Thanks for the tips. The 3C589 cards are pretty easy to come by on the 'bay.