I'm going to guess they won't heat it for you, but you might find a sympathetic FA who is willing. They heat things in ovens mostly so if your item is wrapped in foil and oven-safe...well, it'd be off-the-book to do it but I have seen FAs heat their OWN brought on board food before.

One thing you might consider to increase the shelf-life of something that otherwise needs refrigerated - freeze it. I do this with sandwiches all the time. Pop them in the freezer and by the time they thaw and I'm ready to eat them on board they're still cool and food-safe. Wouldn't work for a 14-hour flight, but works for the first 1-3 hours. I do egg sandwiches this way on early morning 6am flights, often.