Hello Stan,
unfortunately, I'm neither an OF expert nor a programmer, but I would guess that the best way to make GRUB boot Mac OS and Mac OS X would be for it to do whatever yaboot does.
I believe all yaboot does is ask OF to boot to the partition containing the original bootloader of OS X. When we dual boot OS X and Linux on a PPC Mac, we just shrink the OS X partition then put our own partitions after it, and tell OF to boot to the new Linux boot partition instead of the old OS X one. When you tell yaboot to boot to OS X, all it's doing is telling OF to boot to that original OS X boot partition. I don't know if GRUB has the ability to do that, but it would certainly be useful.
-- Ben Westover
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