From Adélie Linux
Ensure you are chrooted correctly into /target before continuing.
Mount your bootstrap partition (HFS formatted, if on Macintosh) at /boot/grub. Run
grub-mkconfig to create a configuration file suitable for your system, then run
grub-install /boot/grub. Ignore any errors about PReP partitions. Then use
grub-macbless /boot/grub/grub to bless the partition correctly, allowing it to be chosen for boot. You then need to install a suitable
ofboot.b file; simply
curl -o /boot/grub/ofboot.b https://distfiles.adelielinux.org/adelie/1.0-alpha/ppc/grub-ofboot.b to ensure this file is installed properly. This should allow you to boot from the disk.