If found this document:
http://www.swox.se/~tege/macppc.txt

which was very helpful, OF responded well to this command:

0 > boot scsi-int/sd@s:0 -a
RESETing to change Configuration!
bad partition number, using 0
>> NetBSD/macppc OpenFirmware Boot, Revision 1.1
>> (tsubai@mint.iri.co.jp, Mon May 10 19:46:09 JST 1999)
Booting scsi-int/sd@s/netbsd
1856444@0x100000+179560@0x2c53bc
 start=0x100000
Copyright (c) 1996, 1997, 1998, 1999
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

trap type 400 at ff8099b8
Stopped in  at  0x230154:       lwz r0, r1, 0x14,
db>


I seems like I'm really close....

James.