> I think I was in a similar situation before. I am using Haswell CPU
> with its integrated graphics for host OS booted on UEFI board. I have
> VGA arbitration disabled in the kernel. I am using a Radeon 7870 to
> passthrough to the guest, and this card did not have UEFI firmware
> available. I obtained the firmware file for this card online, maybe
> from manufacturer (can't remember), and modded it with UEFI portion of
> firmware from a newer R9 270. Using qemu romfile option with this
> modded file, I successfully passthrough this card to Windows guest
> booting with OVMF. I did not flash the modified firmware to the GPU
> itself, so I have to use the romfile in order for guest to boot with
> OVMF, although it is an option.
AWESOME. At least there is a success case, so it is possible.
I need to dig a bit into BIOS modding communities to get the facts clear about how consistent the results are and how the modding procedure is performed, since after all, due to the ROM size constrain I mentioned early, no one would have been able to try a Radeon 5770 doing pure UEFI Boot on bare metal, so I can't base myself on someone previous experience with my specific model. The only option I have is to give it a try myself. Still, looks VERY promising.
BTW, there was some guy that had my issue in bare metal and actually "upgraded" his real Video Card Flash ROM with a bigger one. Check here:
Sad that it is 15 years old Hardware. Interesing nonetheless.