EDIT 5 (can't edit anymore :( ): I imagine that's what gpg-agent does when working as a ssh-agent, and the ssh server has a public key corresponding to the open

EDIT 5 (can't edit anymore :( ): I imagine that's what gpg-agent does when working as a ssh-agent, and the ssh server has a public key corresponding to the openpgp card that would be the yubikey. Really, U2F seems to do a subset of what PGP can do.