One year on, things have changed :-) Thanks to the patch to prevent the
video from displaying black on black at
  http://www.netbsd.org/Ports/macppc/models.html#5500
no more need for the serial console :-)

Rather than trying to type the nvedit commands while in black on black, one
can enter the patch in the nvramrc box of BootVariables (and click
use-nvramrc?)

The other thing which changed was the ability to cross-build a macppc release
and burn the disk :-)

Unfortunately, I can't figure out how to boot from it. I made an image with
and without the -l option to mkisofs, as the install notes mention

       Open Firmware 1.0.5 and 2.0.x machines will not boot from
       the ISO 9660 file system if you create a hybrid HFS/ISO
       format CD-R. Make sure that it the CD only has ISO 9660 data
       and does not have an Apple Partition Map.

and the Power Macintosh 5500/225 has Open Firmware 2.0.3. AFAICT

  mkisofs -o output.iso -b i386/installation/floppy/boot-big.fs \
           -c boot.catalog -l -J -r -L /cdsources

as per http://www.netbsd.org/Documentation/bootcd.html won't know about an
Apple partition map so should be fine? (By the way that page mentions
ofwboot.elf, and I saw somewhere else that ofwboot.elf is obsolete as
ofwboot.xcf deals with everything ?)

devalias scsi = /bandit/ohare/mesh
dev scsi ls
/sd@0,0      /st@0,0       (what is st? the hard drive is on ata AFAICT)

so I would guess that

boot scsi/sd@0:0,OFWBOOT.XCF MACPPC/BINARY/KERNEL/NETBSD_I.GZ

should do the trick? It just gives me DEFAULT CATCH!

dev scsi/sd@0:0 words includes open
boot fd:0 works
0 > dir scsi/sd@0,\ DIR method failed
 ok
STACK UNDERFLOW!


As boot fd:0 works, and can read cd0 at scsibus, we're off. (It seems that
the Kingston KNE40BT tlp0 card still isn't happy with all its receive ring
overruns and timeout errors..)

Cheers,

Patrick
( no need for a serial console :-) )