lipos shouldn't get warm while charging, so heat won't be an issue.
You could charge NiMHs in there I guess....might get a little too warm too early maybe.....I've read some (I'm sure)
controversial things about NiMH charging pertaining to heat, but I won't even mention them unless I can find where I read it. (try not to turn the thread into a "I know more than you, you're wrong" thread).
I've seen a video of fire blowing right through the cheap HK lipo sacks. Pretty sure I can find that one....
http://www.youtube.com/watch?v=kENxy73h-w0
I still use them though....probably would have been worse without the bag, and for a couple bucks I might as well.
edit: of course that vid was made by someone trying to sell an alternative to lipo bags, so take that however you want.