I don't know if anyone here is interested in this, but some time ago (1998) I started writing a pc-compatible bios for fun, but never finished it.
So in case someone is interested in the little code I wrote you may find it at http://ranmachan.dyndns.org/~ranma/freebios-0.0.1.tar.gz (~67KB) ... [As long as my system is up, which should be nearly 24hours/day, and my internet connection and the dyndns.org dns updates are working...]
I stopped because I had trouble figuring out how to enable SDRAM on my Pentium Mobo with Intel chipset... However for an older 486 Chipset there were specs in an old c't magazine, so I was able to boot into dos with this piece of code... (Don't know if it's still working, I did not try it recently... The floppy code is buggy IIRC... No HD support)
Of course this probably doesn't help a bit with openbios development, but maybe someone will find it interesting to look at :-)
GPL license, no reverse engineering of propriatary BIOSes was involved, I had to go with only a few books (Basically "PC Hardwarebook" and "PC Programmers Guide to low level Functions and Interrupts")