Carbon is lighter than oxygen, so their plan would work if they brought fully saturated hydrocarbons including ones so long-chain that they were solid at room temperature.
You just described coal. So, you're suggesting taking coal on a backpacking trip along with a very heavy canister of hydrogen. To make water? Anything else you want to do to help destroy the environment you're backpacking through?
You realize this whole thread started with the concept of dehydrated water, right?
