The result of thinking a little, but this could well be possible. Synopsis below. Mechanics merely suggested instead of elaborated on, because I don't wanna step on any coders' toes:
Can't help but feel this dungeon could be unlocked by a quest called "The Dark Side of Tycoon", and given by a dwarven engineer over in Tomra called Shanitt.
"You've heard the story of Shanitt, a serious-minded but metaphor-heavy Dwarven engineer who was there when the Crystal Tower was researched, who'd taken an interest in the remnants of the Tycoon, the artificial enigma who brought the Tower to the First, but was unable to salvage any parts from it courtesy of increased activity from the local machina and monsters from beyond time, tougher then what has been seen before. He's asked you to round up some of your strongest friends and accompany him back into the depths of the Crystal Tower to retrieve the Tycoon's parts, and he's promised to make it worth your collective while..."
(cont'd)