Airlines are pretty wary of random foods from random people which may not be packaged properly and either leak or even catch fire in the (fairly powerful) aircraft ovens.
You can get hot water from the crew, if you want to bring along something which can be rehydrated and warmed yourself (such as the sort of food made from dried grains such as couscous, or dried soup mix - stuff that passes the TSA checkpoint). I've seen people bring their favourite instant noodles and so on (put its sauce packet in your liquids bag when going through security).

Other than that if you can't or won't eat the airline provided food, I suggest bringing along shelf-stable ambient foods to eat.

A polite request to warm things up will not get you into trouble, your request will simply most likely be declined. Of course, going non-linear on the crew over such a request, as with anything else on board an aircraft, will go poorly for you. I'm confident you would not do that.