Find a motherboard that uses the Via 8501/686a chipset (or Cyberblade i7), that has a newer phoenix bios, grab a copy of that bios, and give it a whirl. If it works let us know, if not, let us know. Chances are, none of us here with the skill have the time to write a complete bios (from scratch) in any reasonable amount of time for free. Never having written a bios before, I could guarantee a minimum of a months worth of my billing time, and at $75/hr, I doubt very much that you'll be able to find enough people interested in a bios that much..
Other options are LinuxBios, TinyBios, and OpenBios. TinyBIOS is where I got the code for the floppy controller bios from, to me it is not very clear or neat, is difficult to follow, and currently does not have support for booting from USB or CD. LinuxBIOS looks promising, but as yet has no plans to support booting BIOS dependant OS' such as DOS and windows. LinuxBIOS should actually be rather easy to port.. but it also (to my knowledge) does not support booting from USB or CD. OpenBIOS I have never taken a really good look at, and I'm not really sure of how Open it is..
There are yet other options. If someone can point me to a document that specifically describes how to make a USB boot bios, I would be happy to write a boot rom that enabled such. It would not be part of the bios, but would be a seperate chunk of code that executes when the bios is ready to boot; similar to the floppy boot bios, the CD Boot Bios, and the interleave enabler.
If anyone would be willing to get me a copy of a book that detailed all of the software interrupts needed for IBM compatability, I would be happy to take a look at it and see how much work would be needed to convince tinybios to be fully compliant, and/or how much work it would be to make a DOS bootloader for linuxbios.
There are likely further options that I didn't think of, If you have any please post them.