Hit a stumbling bock on the PPC, though as it seems that the varargs function has different behaviour on different platforms (passing parameters in registers or on the stack).
Well, you pick one ABI for AROS/powerpc and stick to it. Be it System V, PowerOpen, or something else. There is no problem here.
the best option would be to remove it from the AROS core, but apparently that would take a huge amount of work
That's just insane. Varargs works just fine with PPC.