Tin cans have a lot more food-to-area ratio which means that a frozen tin would take a lot longer to thaw out than an MRE. With an MRE, you would probably need two heaters - one to thaw it and one to heat it to eating temperature. But my other idea has some merit - just put the MRE or tin can on the engine block to thaw and while running the engine to heat the passengers, it thaws the meal as well.