I have a G4 desktop running OS 9.2 & OS X on separate partitions, and a G3 Pismo laptop running Tiger.
Today I was backing up to an external (firewire connected) hard drive. After I make a backup copy, I always check that the backup drive will boot. Out of curiosity, I selected the Pismo partition to startup on my G4. I was surprised that it booted on the G4.
Not sure if this info is useful for anything, but I thought I would pass it on.