I use a combination of Yubikeys and OpenSSH certificates to authenticate in my environment. But when I want to use my ssh-agent to bring my keys and certificates with me I have some problems. I cant find a way to actually add the certificate to the agent when my keys are stored on a PKCS#11 device.
This seems to be handeled by this ticket: https://bugzilla.mindrot.org/show_bug.cgi?id=2472
BTW You can use certificates in ssh already using keys stored in an agent or token. Certificates are grafted to external keys at authentication time if they are available.