I would check with the airline to see if they have suggestions. Even if you don't want to eat their food, they can let you know if they are able to heat food for you and maybe they have ideas that other travelers have used. (in my limited experience, international flights have been a lot more customer-friendly than domestic, where they treat you like cattle.)
If you search for "self-heating meals" there are products that heat themselves. I have a feeling it will end up being expensive and uneaten, but you could give it a try.
You could try ramen-type meals. I've seen higher-end options at places like Whole Foods, and you can add hot water to those and let sit. Same with Campbell's dehydrated soups.
You could also look at some of the newer vacuum-packed convenience meals from the grocery store that don't require refrigeration - for example, the rice dishes that are in vacuum-sealed bags. I keep seeing new products, but can't think of other examples offhand, but you could put the bag next to your skin to warm it to body temperature, at least.