
The only way to be able to 'attune' to Bahamut would be to be where it fell. In battle. Shortly after it fell. Unfortunately, lore wise, only the Warrior of Light managed that bit. Bahamut has not been summoned otherwise.
That being said, the Dreadwyrm Stance does make use of the tainted aether within Eorzea, so that at least works.
That being said, the Dreadwyrm Stance does make use of the tainted aether within Eorzea, so that at least works.