Originally Posted by eturowski

You can print your boarding passes to a PDF file and then send them to yourself, then just open them when the FA comes around with the beverage cart.

YMMV, though. In my experience, FAs are more likely to either look at the coupon and wave it away or just look at it dumbfounded and wonder what to do with it. I've never had one actually try to scan it or type in a number, which is nice, because then you can use it again.