An excellent idea but may not be practical. Each train say has 1000 passengers of which 50% feel like having the meal in the restaurant car. then 500 people have to be fed. An extra coach can house only around 50 diners with tables. So there have to be 10 rounds. If the 50 people of each round take 30 minutes, then 5 hours will be needed for the meal. So if meal service starts at 7pm it will end at 12midnight.
I discussed this with a very senior railway guy, he told me that if there is scope for an extra coach we would like it carry people as we are a country of Waiting Lists.