For future reference, it seems that some versions of Open Firmware can boot
from ISO 9660 format volumes and some can't.  Here's what I know:

OF 1.0.5 (PowerMac 7500), can boot from a pure ISO 9660 volume (without any
Apple or Microsoft "extensions").  You *must* use capital letters to
specify the file to boot from.

OF 3 (PowerBook (FireWire)), cannot boot from any ISO 9660 volume (though
it works with a "hybrid" HFS/ISO disk).  You can create a hybrid disk with
a MacOS utility like Toast, or with one of the packaged CD-ROM mastering
utilities.  The error it gives is something like:
method <#blocks> not found, ihandle ff9c9b40 phandle=ff851ef8 open
cd:ofwboot.elf: unrecognized Client Program format
unrecognized Client Program format

Can someone clue me in on how well OF 2.0.x and OF 2.4 machines handle ISO
format volumes?

Thanks,
  -- MW