I have done this, even asked for meal from economy -- once was given a main course from business when I was in economy -- but it is about being able to eat. I'm vegetarian so when there is a mix up on my special meal or I switch flights or get upgraded, I'll just ask if there are any options for me. Worst case a tapas box will get something in me, but it wouldn't my first choice. I've also offered to happily buy a box from economy and never been charged.