VBIOS is often not necessary for running a GPU in the OS. The amdgpu driver can POST a GPU by itself just fine.

Still… X86EmulatorPkg allows running an amd64 VBIOS in UEFI on an aarch64 machine :)

AFAIK the bigger problem on embedded boards is half assed Synopsys Designware host controllers. I have a Radeon running on my Marvell MACCHIATObin, on FreeBSD even. But from what I've heard the Rockchip RK3399 has a worse version of the controller, and people trying GPUs on the ROCKPro64 saw errors related to not large enough BAR space or something.

UPD: yeah, someone in the thread mentioned BAR space issues wrt NXP i.MX SoCs, that's probably what's happening on Rockchip. Would be amazing if the Broadcom chip in the Pi turns out to be the one with enough BAR space! :D