I've pondered the same. That's why I now carry a metal cup! Also heavy duty foil (not the rolled kitchen stuff, a foil turkey roaster pan cut up). But first I'd go for the water I always carry, then the water purification tablets, and finally the mini-filter straw I carry too.

In other words, I thought about your same scenerio, couldn't figure out anything terribly efficient to handle it, so I gave in and decided to carry the minimal stuff that would stop such a situation from unfolding (hopefully)!