I got a few replies (thanks to Soren Jorvang, Dave Huang, John Kohl
and Brian Grayson in arrival order), most of which suggested using
dosboot.com.

I tried that, and typing either "dosboot" or "dosboot netbsd" both
resulting in the machine locking up, with only just a brief flash of
hard disk activity (which I guess is dosboot loading).  For reference
again it is a PII 400, 64MB ram, 3c905 and 3GB IDE disk.

Also, I don't want to repartition the box ("they" still haven't finished
setting it up supposedly, that's why it's only got Win31 now), and
adding extra hardware (disks, zips, etc) isn't an option.

For now, I've got a kernel that fits on a floppy without compressing
(1337kB!), so I'm just booting of that and swapping to a 128MB file in
/dev.

Already had my first panic (pmap_steal_ptp: failed to steal a PTP!) -
I'll PR that tomorrow morning.

Thanks to all again for the replies.

Simon.