One might imagine a web service that rewards people for creating such unsolicited ads in a financial way based on the effectiveness of those ads.
In essence, instead of having a company tightly control its image, it would leave marketing to people interested in trying out their own ads independently. And those who succeed in helping the company financially are paid in return.
You could run it like a startup model combined with the recent doritos campaign (big cash prize for the winners). Throw around small (seed) money to anyone with a half decent idea...give out "A-round" funding to campaigns that focus-group well to expand their ideas...and entice everybody onwards with a big cash prize (acquisition!) for the campaigns that succeed.
You would need a very solid plan for determining "effectiveness" of the campaigns. Something that the advertisers trusted enough that they are willing to guarantee payment when you determine their campaign has been "successful".
Pretty simple site with a client side where advertisers can submit the broad outlines of what they're looking for, user side for submissions. Sit in the middle and skim off a little cash as the money flows past...
Very scalable. Advertisers are desperate for creative new approaches. Billions for the taking.
In essence, instead of having a company tightly control its image, it would leave marketing to people interested in trying out their own ads independently. And those who succeed in helping the company financially are paid in return.