You are free to implement any filesystem driver, and either load it in your custom implementation of EFI, or from an EFI image stored on a FAT partition. The EfiFs project provides many drivers under a GPL, including exFAT, Btrfs, ext2-4, NTFS, ISO9660 (cdfs), ZFS and also HFS(+).
You are free to implement any filesystem driver, and either load it in your custom implementation of EFI, or from an EFI image stored on a FAT partition. The EfiFs project provides many drivers under a GPL, including exFAT, Btrfs, ext2-4, NTFS, ISO9660 (cdfs), ZFS and also HFS(+).