Hey Tim,
I'll let one of our support engineers comment on the SSH keys in LDAP, but in terms of support for Yubikey, that comes out of the box. All you have to do is enable U2F authentication in the Gluu Server and any U2F compliant token, including Yubikey, will work for 2FA.
Docs for U2F are [here](https://gluu.org/docs/multi-factor/u2f/).
Thanks,
Will