Hi folks,

OK, I finally managed to install a single-disk system from scratch.
The key points are that you must boot with a root in a swap partition,
and when partitioning the disk in sysinst you must MANUALLY change
sysinst's partition table so that the b partition is in exactly the
same place, and exactly the same size (doing this seems to require
specifying sizes and offsets in blocks.)

Briefly, the instructions should be:

[first, get machine to a state where you can dd the diskimage to a disk
(e.g. net boot it)]
disklabel -W rz0
gunzip -c diskimage.gz | dd of=/dev/rrz0c bs=10240
dd if=/dev/rrz0c of=/dev/rrz0b bs=1024k count=32
halt
boot -f rz(0,0,0)netbsd n
rz0b
[return twice for dump partition and fs type]
[return for /bin/sh]
mount -u /dev/rz0b /
TERM=pmax; export TERM
sysinst -r ""
[choose full install, say you want to manually partition, say you want
to use blocks, set a partition to size 65536 offset 0, b partition to
size 131072 offset 65536, d partition to size remainder of disk,
offset 196608]
[continue with normal full installation procedures]

I hope that's clear enough for someone to turn into real
documentation, I have to run right now (already late) so I'm not going
to even proofread it right now...

Cheers,
entropy

--
entropy -- it's not just a good idea, it's the second law.