This would make a good way of restricting access to the deeper catacombs, but we'd need a decent justification for why you need girls to beat the boss monster. And probably a more interesting combat mechanic. Otherwise it's going to be a case of select every girl in the brothel, send them down the catacombs, heal and repeat.
On the topic of cave dwellers or boss fights, here are a few suggestions:
Set up conditions on the cave dweller, something like this. The Kraken could only be damaged by the Spear of Poseidon. But the game only has one (maybe 2?) Spear of Poseidon weapons in the game. So only the girl the weapon is equipped on, could do damage to the Kraken. The advantage to this system is that it would be relatively quick and easy to add in to the current code/combat system you have in place already. I would prefer not to handle cave dweller fights like this since.... it's kinda' lame.
A second suggestion that would require a little more work but could still be implemented with the current combat mechanic:
Make the cave dweller vs. girl fight, series specific.
For example: One cave dweller could be "Grimmjow" from the anime Bleach. So the only girls that could battle him are the girls from the anime Bleach. Such as Matsumoto, Rukia, Inoue.
A final final suggestion on cave dweller battles. If you're feeling ambitious.
There is an H-game called: Princess Waltz
http://www.erogereview.com/2008/04/30/princess-waltz/ This game has a "card battle system" for handling all monster battles. This may go WAY above and beyond the scope you had in mind for WM but If you were interested in making a second combat mechanic for the game, an in-game card battle game might be cool.
The advantage to having the player do boss, cave dweller battles like this, is that it gives the player some sense of having control of the actual outcome of the battle. Instead of just clicking one button and letting the computer generate two random numbers, deciding who wins and loses.
I recall on an earlier forum, the idea of an Arena for girl fights. This same card battle system could be used there as well.
If card battling seems like more work than you want to do, you could consider posting a request to the Newgrounds.com board. They have all sorts of Flash game makers over there. Maybe someone would be willing to "whip" you up a snazzy little card game.