In a couple of instances I've asked even using the words "can I buy something from the buy on board menu?" and had them serve it to me free, but that was in lieu of a J class meal. I forget which flight I was on but the 2 options were totally unappealing to me but I did wanna eat. Different situation, but as others have said, it never hurts to ask, you just have to be prepared not to argue if you get told no.