Maybe not on amd64, but GRUB **boots** from HFS on NewWorld PowerMacs
(G4 and G5 and IIRC also tested successfully on G3 Blue & White).
Anyhow, from what you described earlier I assume it's more like the used
x86 firmware provided by VirtualBox can't load GRUB from a HFS
partition, which is pretty much expected. GRUB itself has ro support for
a lot of file systems (check for example [1]).
[1]: http://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/fs
Now I feel a bit stupid, I googled but didn't think of checking the source :-P
But that should make it possible to have the slightly "hacky" solution to put /boot on a HFS partition, which should be bootable from Open Firmware.
Is the solution with only having grub.img more secure, or is it just to make it easier to change for those who already have systems up and running with yaboot today?
Regards
/M