Trav,
Wow, that's cool! :-0 (pun intended) I'm just starting to mess with a heat pump and sure wish I had a dry well to play with. But heat pumps aren't cheap so...
I'm not sure humidity would be an issue. It's in summer that you'll be using it after all. So don't borrow trouble, set up a duct and a fan and see if the moisture is an issue. If it is, you could maybe drop a little cement down there to reduce it.
Jonathan