I wasn't able to boot a G3 green/white powermac with only ISO9660 as a filesystem on the CD/DVD. OpenFirmware generated the following error when trying to load yaboot it also doesn't read any files from the CD/DVD at all.
load-size=1000 adler32=b265d7b0 unrecognized Client Program format
BTW manually booting a yaboot file on a HFS CD only worked when declaring the full path.
Instead of the normal code to boot the CD which in my case booted the internal HD.
Swapping the CD/DVD/HD devices on the IDE / ultra-ATA motherboard connectors makes OpenFirmware identify my cd as cd again instead of HD.
I will also try booting from FAT, and try some other ISO9660 combinations made with mkisofs. Running the following code in OpenFirmware shows that HFS/ISO9660/FAT are supported...
dev / ls