How to Trick a Mac into Booting EFI Grub

Make sure that /dev/disk0s1 is FAT formatted, has EFI grub installed in EFI/grub/, and its GPT partition type is EF00 (EFI System Partition).

# mount -t msdos /dev/disk0s1 /mnt
# bless --mount /mnt --setBoot --file /mnt/EFI/grub/grub.efi
# umount /mnt