Boarding cards or manifest or a divide are the only ways I know of. Other airlines do it so it must be possible. I remember EasyJet had a coupon printed for discounted drinks a while back. Perhaps those entitled to a freebie could have a coupon printed on their boarding card which could be scanned by the crew. I recall BMI used to give a free snack if you were a FF and it was a matter of showing the Diamond Club card.