HammerGuy:
SpoilerI wondered when someone else would refuse to take the prize for 324/324 at face value and look around. There have been a few things I've wondered about that, but since you're the only other person I know of to access the postgame, I'm not sure if you'd have any more answers about some of the bigger secrets of the postgame than I would. Like, for example, the clue at the end of BS 2 Level 9.
Spoiler
I saw an old man in BS2 "Registered" that commented on the player having already done the final levels (in this quest being the FRAGMENT levels) Beyond that it seems there is no more post-100% dungeons beyond OUT1+2 and BS1+2; with the end of each level 9s just going back to the entrance..
The only other notable out here is the Map/Compass Guy::
Heaping piles of rupee rewards if you have all 124 Compasses and 126 Maps. (The Outland Level 9s have no compasses)