Incentives could be something like being a named in the ad, owning a print of the ad, owning the original art for the ad (that sort of thing).

If we went with SilverWook's idea for a billboard or two the incentives could be to be on the poster, design the poster, keep the art or a copy of the poster. There could even be a pamphlet about the billboard/ad project which would fall under the Kickstarter publishing category that way the billboard/ad could be vectored into the project cost.

Considering how collectable these things can be I could imagine some people going even for that.