according to man xorriso, that would looks like: -boot_image "grub" efi_path=bootefi.fat32 platform_id="0xef"
where bootefi.fat32 would be a file in FAT32 containing both: /EFI/BOOT/BOOTIA32.EFI /EFI/BOOT/BOOTX64.EFI ... I think. Will now look atAndrea G. Monaco's post on grub-devel
