i thought this pickle was because the installer wouldn't handle it. quite likely grub would be happy enough inside the raid. if you feel safer or happier with grub inside the raid, you can probably put it there. but if you (or the update process) create /boot inside the raid, bootup will still come from un-updated partition outside the raid, until you "grub-install" for your new /boot. or if you want to leave your boot partition where it is, again, then you want to get the updating to happen with the copy the gpt-mbr points to, iow have it in fstab and mounted as /boot.