So I did a small 2 day camping trip and I bought a couple camel back pouches. We ended up hanging them up and taking off the bite valve and using them as a dish washing helper. It has an easy to use on/off switch. Will the plastic inside be fine with hot water (not boiling) so a shower can be taken.
I guess it depends on how hot and how long. I rinse mine out with straight hot from the tap but it's not like it has a long time in the bag.
Walmart has "Solar shower" bags for a couple of bucks that can handle hot-hot water... and more of it...
Have you asked Camelback? Their website makes them look like they are pretty sociable, and companies often are happy to answer questions like this about their product. (http://www.camelbak.com/)