If you're looking to try that. Be careful with beans, either get them pre-cooked or make sure they're a type that's safe to eat without boiling.

https://en.wikipedia.org/wiki/Bean#Toxins

I actually hiked with a person who "cooked" or rehydrated meals by putting the ingredients in a sealed plastic bag and then putting that in his underwear. Seriously.

Ah, the infamous crotch pot slow cooker.

Perhaps a clever, yet obviously comedic, idea. But which foods will cook at a warm temperature that wont at a cold?

There is no cooking involved in those cases, just rehydration. It just happens faster at higher temperatures. And it's less unpleasant than eating ice-cold.

Yes, I should have used prepare in place of cook, as cooking requires heat.

Wouldn't bacterial growth be a concern in a wet, warm environment for an extended period? Even if the food was bacteria-free, the container can pick things up from the environment. Even if it's low-risk, on a long thru hike I wouldn't want any risk of stomach issues.

I believe all the food is sealed air-tight.