Hi future me, just a reminder that you forgot this last time. But booting a mactel doesn't need special "bless"ing. Just remember to install grub2 properly.
grub2-install --target=x86_64-efi --efi-directory=/boot --removable --modules=part_gpt
Also, grub2 doesn't seem to come with vbe.mod anymore. So on Calculate Linux, edit /etc/default/grub and change GRUB_VIDEO_BACKEND="vbe" to something sensible. Perhaps "all_video". Then re-run grub2-mkconfig.