I accidentally installed a FreeBSD drive in my MacBook. To my surprise, it just worked. If you install a boot drive with the "usual" PC disk partitioning the Macs will boot in what seems to be a fairly complete BIOS emulation.
How far Plan9 gets is another story, but my guess is that even if it sees the disk controllers properly, it's not going to like the NIC(s). Don't even try to install it on a Mac. Instead, pull the drive and do the install on a normal PC, then stick the drive back into the Mac. This might give you a fighting chance, but it's inevitable you'll be doing driver work before long.
