Perhaps you could ask the FA when initially boarding and getting seated...

"I am unable to eat the First Class menu items and would rather buy an item from the Y Eats Menu? Could you accommodate me please?".

By doing this in advance, it allows them consider how they can "steal an item", ferret it up and plate it for you. Once they have started serving, it may make it a bit more difficult.

I watch a lot of those "famous chef fixes doomed restaurant" shows and understand the issues here. Just as you don't want uncooked items to be in the same refrigerators as cooked ones, The FA may have to specially wrap the Y-class menu items to keep "cooties" out of the first class kitchen. :-)

When booking, do you have an option of asking fro "special meal"? Not traveling in FC I am not aware of how that works anymore.