subUA 757s have never had ovens in the economy cabin. Hot meals were provided via the 'new-tech' carts where part of the meal tray was actually heated. Those 'new-tech' carts are no longer boarded at all on the 757s - and I doubt it is realistic to consider using them to cook the cheeseburgers at this point. UA will have to re-do the galleys if they want to offer this option.

Because the dead-TED 320s are not strictly isolated to particular routings against the 319s and other 320s (they are in the planning phase, but that is totally thrown out the window in the operation) it isn't feasible to serve anything hot in Y. The dead-TED 320s have only a single, small oven in First Class and on any meal flight the space in that oven is completely maxed out between the First Class meal offerings and the Pilots meals.