> Instead this should have been designed around something I can throw in a campfire with some sort of protective spiral metal cabling to snake out of the fire to a box with a USB connector.
Your problem is that the sink capacity of your device is closely related to its mass so unless you let something change phase (for instance, water evaporation) you're going to run out of capacity to store the heat before you have your phone charged. This will result in a lack-of-temperature-gradient which means you will no longer produce power. Compare with a Peltier element that is kept equally hot on both sides or equally cold.
That won't work, see:
http://en.wikipedia.org/wiki/Thermal_efficiency
Your problem is that the sink capacity of your device is closely related to its mass so unless you let something change phase (for instance, water evaporation) you're going to run out of capacity to store the heat before you have your phone charged. This will result in a lack-of-temperature-gradient which means you will no longer produce power. Compare with a Peltier element that is kept equally hot on both sides or equally cold.