Watch your waste: The problem with airline food and packaging
Pre-order system would be effective, if it's done on short distance flights with cheap meals.
What do you think, if airline carrers want to sell food on board with rip-off prices?
Some people may bring foods which are available with less money. Then, total weight of each airplane would be heavier than usual.
So, the best way to make airplane lighter is to install pre-order system, and to serve it with cheap enough to discourage passengers to bring food onto flight.