How much does coreboot care? It needs to support the specific motherboard+CPU, but after that it's not like coreboot needs to handle most hardware. Excepting of course storage (USB, SATA, etc.), but that should be equivalent to laptop difficulty, and possibly network cards for network booting, which I grant might well be harder in desktops.

But mostly I'd expect coreboot to need fairly minimal hardware support and then just pass off to the real OS with proper drivers.