I wouldn't think anything about it....I'd appreciate the drink. My friends and I know the game - we've all been on both ends of an F upgrade list. No one would think it pretentious....it's not much different than the guy who had the best night at the craps table buying the first round of drinks at the bar.

The only concern I'd have with doing it is logistics...sort of an awkward/unusual request to make of the FA in F, and he/she probably has to go communicate with an FA somewhere in Y and hopefully get the message to the right place, with your credit card, and then get your credit card back to you. A lot of work for a drink.