Before connecting each client needs to be set up with (1) its own private key and (2) the server's public key. The server also needs to have each client's public key. Once you have securely shared this information out-of-band, there cannot be a man-in-the-middle attack because both sides know the expected public key of the other side, and can prove ownership of their own public key.
And no, out of the box there's no equivalent to "trust any cert issued by this CA and lookup the username in ldap based off of the cn". Enterprisey auth is left as an exercise for the reader.
Pretty much the same way vanilla SSH does, without the trust-on-first-use bit.