OK, after applying the pci.c, obio.c, wdc_obio.c, and ukbd.c kludges,
I get as far as:

wscons: wskbd1 glued to wsdisplay0
trap type 300 at 15c300
Stopped in swapper at 0x22acec: lwz r0, r1, 0x14,
db>

I can type things now though, but it turns out to not be useful.
Asking for a stack trace gives:

db> t
at ?+ffffffff
at ?+ffffffff
at ?+ffffffff
at ?+ffffffff
at ?+ffffffff
at ?+ffffffff
at ?+ffffffff
db> 

Oh, and I don't think it found a wd0, even though it did find the cd0.

-- Dan

PS - Not especially related, but does the PowerMac Open Firmware have
an equivalent to the Sparc Open Firmware "power-off" command?