I have done this many times. Right after the fasten seatbelt sign gets turned off go to the back and ask the flight attendants before they get busy with the drink cart. I ask them if someone buys a drink could I use one of my drink certs to pay for it and their cash would pay for the BOB for me. They can't do it otherwise because they need to have cash for the BOB boxes. I have not been turned down when I asked in this way.
Dan