Dine on demand isn't offered although I read someones experience on here and tried it out in J. When they take your order tell them what you want but that you want it later and they'll set it aside. One FA said he couldn't and I told him I'd had that previously and he checked and was fine with it. It's just that I've generally eaten in the lounge or prior to airport so not hungry and don't necessarily want to stay awake for a long service. Sometimes I just want tray service then I'm out. I find dine on demand I end up eating only once instead of twice long haul but not fussed.

Never tried this in PE though. In the distant past I was given economy tray food later in the flight (in Y) because they missed me. I may have moved seats. I can't recall if food is free in PE for snacks could ordering from screen could be an option.