Okay, I've got a little more info.  First of all, the ultrix disklabel
reading seems to be wrong: I should definitely mention, though, that
the Ultrix disk is from 4.4, *not* 4.3.  Could this be the source of
the problems with mounting the partition from the Ultrix disk?

I'm feeling a little foolish now: I successfully loaded all of base.tar.gz
on to a temporarily-Ultrix partition on rz0 (the NetBSD disk) and relabelled
the disk as NetBSD and rewrote the miniroot.  I then converted the
partition to NetBSD with fsck -c 2 (good thing I have a running Amiga
NetBSD system with man pages!) and copied the binary for mount_nfs to
/sbin after nuking a couple of less necessary binaries...

And I'm left with the old "I've got one bootable partition and I want
to resize it" problem.  Am I correct in thinking that I can't try to
write the miniroot to rz0b, since it'll need swap?  And that a filesystem
can't be resized (like from the 5MB miniroot fs to the partition's 64MB)
when it's the boot partition?  Lastly, is it possible to boot off of
partitions that are not a?

As all along, thanks very much for your help,
--
Todd Masco     | "'When _I_ use a word,' Humpty-Dumpty said, in a rather
cactus@hks.net |  scornful tone, 'it means just what I choose it to mean -
cactus@bb.com  |  neither more nor less.'"   - Lewis Carroll