Found him! I knew I had seen Desch's name in FFXIV stuff before it was mentioned in HoH...

Quote Originally Posted by Encyclopedia Eorzea - G'raha Tia
G\raha Tia's bloodline can be traced back to the Third Astral Era, and a figure whose true name has been lost to the ages. Known only as "Desch," Seekers of the Sun such as he were but a minor clan from the outskirts of Ilsabard. During the height of the Allagan empire, Desch was conscripted into the imperial army alongside his few brethren. The Miqo'te somehow gained the trust of the princess Salina, and, and the ear drew to a close, was entrusted with her blood and the secrets necessary to control the Crystal Tower. He survived the Fourth Umbral Calamity, and the members of his family have since carried the royal line of Allag.
So yeah, the founder of the Onishishu probably knew all about what happened with the Crystal Tower and how Salina made sure it could still be opened in the future. You could probably make a good case for the Onishishu being yet another of Salina's fail-safes for making sure something of Allag survived.