UEFI doesn’t forbid you from implementing additional file systems, it just requires everyone to support UEFI-FAT.
iBoot for example supports booting from HFS volumes.
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(+).
UEFI doesn’t forbid you from implementing additional file systems, it just requires everyone to support UEFI-FAT. iBoot for example supports booting from HFS volumes.
And who has that implementation?
iBoot is the bootloader on all Apple products.
But who else can use that?
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(+).