Something else you might want to consider (this is what I ended up doing, since I didn't want to use NFS) is creating installer LiveCD along the lines of http://www.alti.at/knowhow/obsdlivecd/index.php. I wrote a dialog based installer to allow techs to do easy OS installs.
The LiveCD approach allows considerably more flexibility than going with the rd (and you don't have the space limitations among other things) and eases development and testing since you don't have rebuild bsd.rd each time, Just "edit" the source filesystem for (updating whatever scripts) and create the new CD image which you can test under VMWare/Qemu or whatever.... - mdf
Looking into the way bsd.rd is built, I came to the conclusion that cooking a custom bsd.rd isn't desirable. What looks much easier to me is to install from an NFS server. So, have a bsd with "root on nfs" with a full OpenBSD installation on said NFS server and no stringent space limitations, but a full-fledged toolkit that you can work with.
