The last cards that did have a G5 compatible OpenFirmware BIOS were the Radeon X19xx cards AFAICT. The Radeon 5870 MacPro Edition pretty safely only works for Intel-based MacPro machines.
It is also a pretty safe bet that none of the Evergreen based cards work for MorphOS currently (in a PMac G5), at least I didn't manage to run any of those which I have available.
It is a different story for MorphOS supported machines like X5000 which run the BIOS code upfront.
I still need to identify what the actual issue with PMac G5 machines is.
I only managed to run some HD2xxx/HD3xxx cards on my PMac PCIe G5 so far.