> I presume you can get a shell utility to pipe your key in and get that mnemonic version out if you'd like.

Not possible as GnuPG does not expose access to the cryptographic seed.

That said a mnemonic you generate and backup with keyfork is entirely compatible with GnuPG should you still want to use it as a client for keyfork-provisoned smartcards.

You can also use that same mnemonic to generate any other cryptographic keys you want for any purpose, including Age.