grub2 won't autodetect the MorphOS kernel. You have to read the grub2 docs on how to add boot entries manually. On OF-systems the kernel is the 'blessed' file on the small hfs partition.

This should work without hassles as the kernel seems to be (is?) a regular ELF-executable which grub2 is able to load manually. Don't know whether this is true for OS X kernel too.

Have not tried it myself yet, still on my to do list.