Be assured that if you did have a tape drive and OpenBSD on tape that you could probably do it that way, along with that old tape of Richard Stallman's EMACS that you have hidden in the loft somewhere.
Well, I have to say first that I have an SL-C3000 and not a 3100 but I think I can see a path to you getting the full 4Gb OpenBSD and you have already completed step 1
Be aware first of all, you are going to lose SD support and there is no proper Bluetooth in OpenBSD at the moment... finally it's not quite as snappy as PDAXROM with regard to running your favourite games so if Quake is your bag I would suggest that booting into PDAXROM is a good thing.
EDIT: Also you won't see any of that 128Mb Flash when booted in OpenBSD so there's no benefit in having a 3100 over a 3000 to do this other than being able to boot into PDAXROM if you want.
1. Install a ROM on your SL-C3100 that doesn't need the hard drive at all, Cacko and PDAXROM are possible options. Shouldn't matter as long as it is based on the 2.4 Kernel.
2. Ensure that the ROM isn't mounting the drive in any way... you probably want to hack around with some scripts for this - you must at least let cardmgr initialise the drive though. If you are determined not to have cardmgr resident then run cardmgr -o which will initialise the drive then quit.
3. Obtain zbsdmod.o, zboot and bsd.rd from one of the mirror sites. Don't waste your time with 3.8, the Zaurus has come a long way since then, go for the snapshot version.
4. Copy these somewhere on the Zaurus, for example create a directory called /bsd on the PDAXROM install.
5. Use the following sequence to get started....
cp bsd.rd /proc/zboot
6. The system should now boot into an install, get that NIC card plugged into the CF slot.
7. If you want to run OpenBSD on the whole Zaurus then allow the installer to do it... Note SL-C3000 users should be using my initrd and stuff from http://zaurus.daemons.gr/openbsd
and not following these instructions. You may want to delete the disklabel and recreate so that wd0a is a little smaller so you have some swap space... my disklabel looks like this if it's any help (seasoned UNIX guys shudder that I didn't partition further but I know what I'm doing and am not going to overflow the /home or /usr space etc).
# size offset fstype [fsize bsize cpg]
a: 7737281 63 4.2BSD 2048 16384 328 # Cyl 0*- 7675*
b: 262144 7737344 swap # Cyl 7675*- 7935
c: 7999488 0 unused 0 0 # Cyl 0 - 7935
note that partition c always refers to the whole disk space on BSD., DON'T delete that one.
8. The most problematical part is getting a WIFI NIC set up to use WEP against your access point (note no WPA yet).. answer yes to performing additional config and you should drop to a prompt. You can then use..
ifconfig wi0 nwid <myaccesspoint> nwkey <mykeytext>
9. Press Ctrl-D to leave the shell and go back into the installer.
10. Follow the installer until the packages stage, (you will have selected HTTP or FTP as an install medium, no proxy etc. and selected a mirror for the install). When the packages list is presented to you just type 'all' and continue.
11. The rest of the install should complete predictably and your Zaurus should prompt for a reboot at the end.
Now booting into OpenBSD.
You will initially boot Linux, in your case PDAXROM..
Go into /bsd and perform the following steps..
OpenBSD will now boot..
Optionally you can create a script file in one of the /etc/rc directories to do that for you.
When you are basicallly up and running take a look at some of my Living with OpenBSD posts because they will help you get your NIC automatically configured with the AP name and key and various other things.
Hope this helps,