I have built a small signing service that works by a user SSHing in, performing LDAP (password) authentication and 2FA with duo, then injecting a time-limited certificate signed by Hashicorp Vault back to their user agent (although it could be modified to remove the Vault requirement). The UX is very simple (SSH to this address once per day), but the backend is complicated, so if there is any demand for me to put this on Github I am happy to do so.