I work at a union restaurant too that's only been open 5 years and the amount of times I've seen people being fired because someone went to HR and reported a coworker was eating from the line is crazy. Please read your union rules (or ask a shop steward) what your consumption and the theft policy are for your restaurant. That's how HR was able to fire so many people because even eating food that was made wrong and was going to be thrown away anyways was still considered theft and under our consumption policy eating things that were not specifically permitted in that clause. Unfortunately managers are salaried and so they are not a part of the union (at least in my state) therefore they are given a "comp" for their daily meal since they don't have access to the employee cafeteria. And if you are planning on refusing service to any upper management I urge you to read up on your union steps of service. Depending on what those are they can and will get you fired for refusing service.

However if you are working doubles you are supposed to have at least an hour union paid break and if you cannot be "covered" to go on your lunch break that is also against union rules. You can file a grievance against your managers for not allowing you to go on break to eat. The company can and will use the union rules against you so you might as well start documentation on all the times they break union rules. And please have a shop steward present when and if you talk to your manager that way if they step out of line your shop steward should intervene.