On a lark, I downloaded the ISO image of Darwin 7.0.1 for PPC and installed it on the iMac server. Neat. Worked fine, but my Unix is rusty so I barely got past creating a user in NetInfo. NetInfo is a great idea, but if you don't know it, it sucks. That's where I am now.
I've finished the download of OpenBSD and am burning it to a disk as we speak. I'll run downstairs later and try an install.