I've gotten out of the asylum, but I want to go back in to pick up a few items. I can't get any lower past the 2nd floor with Irenicus' lab. Any ideas?

Deathblow

I don't think you can go back into the maze legitly.......but you can always use the MoveToArea cheat of the console command.

And you might have to use the MoveToArea cheat after you pick the stuff up, since it might cause loop when you reach back to Irenicus's lab or Spellhold. If you have the cheat codes enable in the game, when you are in the town press [ctrl] + [x], that'll give you the area code which will be something like ARxxxx, where "xxxx" represent some number.