Just take a farseer. He accompanies the Corsair, as their actions seem to be very profitable to the Eldar race, so they get support. That often happend. or you could just use a Shadowseer. He would be the only Psiker that is let into Commorragh alive.
...What about a Bastion made out of Guardsmen as some form of sick sarcasm?...Or a Fortress of Redemption made out of Dark Angels - Sky Serpent