Woke up quite early with that kernel panic bugging me. I read the documentation and the fix for it was to reboot MacOS 9 (which wasn't as simple as it should, since MacOS X apparently de-blessed MacOS 9's System Folder). Anyway, after reusing XPostFacto, the PowerMac booted with MacOS X! Woohoo!
Well, seeing this OS booting on this old computer is quite cool. Well, apart from some severe usability problems and slowness, MacOS X is an amazing operating system. The problem is: it is slow. It is slow with a G3 600MHz (100MHz of bus) with 128MB of RAM and, obviously, much more so with a dual 604e 180MHz with only 64MB of RAM.
BTW, MacOS X isn't able to use both processors when it is booted with XPostFacto.
BTW#2, XFree86's native driver for Mach64 doesn't include support for the XVideo extension.
Also used the client-side of NFS and SMB on MacOS X, which is quite easy to use.