Just as we used to have smoking and non-smoking sections in restaurants, we could have diet and non-diet sections.
In the diet section, you are only allowed to order so many calories. If you want to get more food, you have to take it home with you.
I like social engineering solutions that involve nudges. Having to decide on a social identity of “dieter” before a meal is a nudge. Wanting to sit with my friends in the diet section is a nudge. Wanting to appear healthier is a nudge. Having to wait until you get home to eat more is a nudge.
It’s hard to stay on a diet while your friends are gorging on a mountain of cheesy nachos. This setup would make eating healthy easier.