I just noticed something in /usr/src/doc/CHANGES-1.6.1 that might be of 
interest:

sys/arch/macppc/stand/ofwboot/Locore.c 1.11
Remove vestiges of the syncicache() call. This change makes 
OpenFirmware 2.4 machines actually boot.

This seems to be a potential fix for the problems that have been 
encountered with this install. If you need an iso of NetBSD-1.6_STABLE 
with these fixes I could probably create one for you this weekend.

Bryan