This is a tough one. A lot of places don't want to handle food that comes in from outside for all sorts of liability issues.
While they may bring a knife to cut it and bring butter, I don't know that they would be willing to toast it. It seems like a minor point, but they are then taking food that they know nothing about and putting it with food that they are preparing for other passengars.
For example, food preperation places have to be very careful with some allergies. Peaple with peanut allergies are very sensitive and some cannot have food that was prepared along with food containing peanuts. They do not know if the bread you are giving them contains peanuts or not and they really can't just take your word for it.
I don't know if they would or not, but I can see why they might not be able to.