Crazy promotion idea. Instead of on bank notes, write the keys (and QR's) inside of a few candy wrappers of a particular brand of chocolate. Make it for a substantial amount of btc value, not just dust, and only a few wrappers. It would be like Charlie and the Chocolate Factory, but in real life. What a huge marketing boon for that chocolate company, they would want the business frenzy and therefore would fund the large reward.