Edit: you do realise we're talking about the graphics card firmware, not the motherboard's, right? The Mac's EFI has a BIOS emulation layer for bootcamp already, so there goes your clean break.
You're kidding, right? It's some X86 code that runs at startup, and you think it's somehow limited? Some motherboards ship with a web browser in the BIOS, ffs.
I realize there is a BIOS emulation in current Intel Mac EFI Firmware. Apple added this to the first Intel Macs via an EFI update as it was required for Boot Camp. This is because at the time there was little support for EFI in Windows. I believe Microsoft wanted to move to add more EFI support but hardware manufacturers are reluctant. Sure BIOS works but it wasn't designed with todays hardware in mind. Apple didn't have to worry about legacy support for OS X as there was a better solution available that would give the features required so they used that.