Why are you using legacy BIOS? It has been obsolete for a long time and is disappearing https://arstechnica.com/gadgets/2017/11/intel-to-kill-off-th...
If the BIOS is sufficient and simpler... why not?
If it were possible would you use the BIOS?
I can see that secure boot is compelling to the right audience, but it doesn't add much for me.
Not a lot of advantages in UEFI over petitboot, though.
Edit: here it is. https://github.com/andreiw/ppc64le-edk2 mocks the presence of ACPI tables using the equivalent OPAL calls.
It's simpler. That's sufficient reason for me.