I've made some progress in that I've got Cacko to install and boot now but no OBSD yet. As I thought, I had to first partition my MD using the pdaXii13 installer's (for the C3000's) fdisk to successfully install and boot Cacko.
Whereas under Linux and most other OSs you can do all your partitioning with just fdisk in most cases, it seems you have to use both fdisk and disklabel under OBSD. I found the official Zaurus install instructions a bit confusing when it came to the partitioning section so I hoped I could just follow the guide you posted -
http://www.planetofidiots.com/zaurusDON'T! (if you're joining in with the necromancy)
Despite stating it is using Qropia to boot OBSD, it says that when you get to the partitioning bit, choose 'whole' ie use the whole disk. I thought it sounded a bit risky to Qtopia but I did this, presuming it would know just to use the whole of the 3+ GB's worth of free space on my MD. OBSD seemed to install fine but when I rebooted it only got as far as:
EXT3-FS: unable to read superblock
mount: Mounting /dev/hda1 on /hdd1 failed: Invalid argument
HDD1 Error!! start check battery !
So it looks like I'll now have to re-install Cacko and then try OBSD again but this time using a custom partitioning scheme