Yeah, as said, throw a piece of sheepskin over the back. Does her *** also get hot on the seat? If so, sheepskin works there also. Make sure you have the furry side up.
EDIT: I'm talking about the furry side of the sheepskin.
I hear duct tape works well also........when the wife complains just put duct tape over her mouth...problem solved! This works for all kinds of problems too...not just sweating. If you purchase the 250' roll that's a lot of complaints
It's not hard to make the tour pack removable. You just need the 4 point dock, a backrest with pad and the antenna relocation kit. Unlike some of the other posters I believe that she would get more airflow w/o the pack.
I had the detachable tourpak set up on my 2001 FLHT but hardly ever used the detachable backrest. My wife said she felt more comfortable and safer with the tourpak. I'd try the beads or sheepskin before I spent the money for the detachable set up.
My Grandmother had a saying, I felt sorry for myself because I had no shoes...until I met a man with no feet. Take the backrest off for a couple of rides. She'll want it back on and no more complaint's...LOL