You probably already know, but here's my list, new references welcome:I may some day write my own custom firmware for these cards, which of course I then would make open source.
Designing PCI Cards and Drivers for Power Macintosh Computers
Apple Technotes 1061, 1062, 1044
Firmworks' OpenFirmware implementation (never found a copy of their book 'Writing FCode Programs for PCI', see their own page)
PCI Local Bus specifications
PCI Bus binding for OF specifications (and more links)
I've done firmware for my own SBus device, OpenBoot/OpenFirmware & Forth are not the easiest to get into but it's doable. In my experience, the actual OS driver is the more complex part (the firmware loading a kernel from non-volatile storage is read-only and doesn't need to be really fast).