The subject of reverse graffiti - cleaning the wall, street or surface as the image of message you want to promote - has been covered several times on this blog. Usually it's some artists that have used it as it is not a crime per say to "clean the streets".
Next step was to take it commercially and a Dutch firm called Green Graffiti have specialized in the art of cleaning the streets product-wise.
Here's an example from the US and Domino's where finders of the stencils could take a picture, send it to Domino's and get $15 voucher.
The new thing here is not the reverse graffiti angle but the engagement part. Do something actively and get rewarded.
Read more about the hook here