I currently host an ssh-only game that logs you in based on your github user. Ie: ssh withinboredom@my-game and it looks your keys up at github.com/withinboredom.keys ... then uses that as the authorized keys (this is all built-in to ssh btw).

It wouldn't be too difficult to extend this to support webauthn/passkeys via a little bit of magic.

Actually... that sounds like a pretty interesting project.