Well, we didn't seem to have any problems, I used the same brand of handwarmers I used to use when I did english riding years ago (winters are cold here, gotta keep those toes warm!) and I never noticed any fumes at any point when I was using them (past or present). Also, the container she was in had a mesh top so the fumes could easily escape if there were any, and lastly, snakes can hold their breaths for an extremly long time (at least an hour depending on their size) so even if there were fumes it may not have even mattered anyway. Just out of curiosity what brand have you heard of having fumes? I would be interested to know, so I could be sure to stay away from them.
Also, I'm not saying these are great for everyday use, but a few hours aren't likely to make a difference.