Well, I have a 5000/25 booted now.  Here's what I did.

1)  plugged a 2G drive into my NetBSD/i386 machine.
2)  cd /usr/src/sbin/disklabel
    ln -s /sys/arch/pmax/include machine
    add CFLAGS+=-I. to Makefile
    edit disklabel.c to remove the #if i386 crud
    setenv MACHINE pmax
    make

Use that disklabel to put boot blocks on the disk, partition it, etc.
(I actually had it partitioned already, using the i386 native version,
but I'm not certain if I was just lucky or not...)

Boot it, and it should go.

It was that sinple.  :)

--Michael