I have seen FAs randomly invite economy passengers to move up to premium cabins if there is space, but only on Asian carriers and not recently. I think this is a good practice: costs the airline nothing (don't give them the meal or other stuff, just seat) and results in excellent odds of repeat business.

The answer to your question, by my estimation, is that you have about a 5% chance of being successful. If you don't mind the likely embarrassment of being caught, then you have nothing to lose. They can't legally bill you for it later.