The celebrities in question would have to be notorious/infamous with blackmailer mentalities, rather than ordinary celebrities.

The closest example I can think of is the traditional transvestite community in India who make a living by showing up in bands at traditional weddings, singing/dancing badly. Some wedding hosts treat them as campy entertainment and pay them. Others find them to be a nuisance and pay them to go away. Either way they get paid. Here they trade their infamy as members of a reviled and oppressed minority for economic gain.

So yeah, it could work, but there would be tricky moral/ethical questions on all sides in most cases. Ashton Lee's Hilfiger example seems to point to the same dynamics.

The general mechanism seems to be one of two things: the celebrity has to be capable of devaluing the brand by positioning it closer to a less lucrative market A, and further away from the target market B where the it has more earning potential. OR make the brand have a minority appeal so that the majority market loses interest in it. I can't think of ways to achieve these conditions that do not involve playing with status, ethnic, nationality or race dynamics in unsavory ways.

This is basically like a big investor shorting a stock on a culture/zeitgeist stock market. I'll leave it to a finance type to figure out a precise analogy.

I think there is reason to suspect that you cannot avoid bringing something like race into it. Why? Because celebrities are usually brought in to position a product in the marketplace with respect to social identities. Social identities work on tribal logic: "me against my brother, my brother and me against my cousin, my cousin and me against an outsider" as an Arab saying goes.

So a celebrity endorsement will create a positioning that defaults to the tightest social identity that celebrity is associated with in the context of the ad.

There are cases where this is inoffensive, but still works the same way. A good way for making kids stop listening to a certain type of music is for parents to start listening to it and raving about it themselves. Here it is generational/age cohort social identities at work.

Some restaurants make kids feel special by not allowing adults to buy certain items/meals (I don't know whether this is to make the kids feel special with a "no adults allowed" positioning or whether it has some obscure relationship to cost; I've seen cases that look like both).

So where no immoral or illegal discrimination is involved, rules preventing a group X from advertising something (rather than payments) can work.

But for a generalized ad network, making this work would be very hard. You'd have to find the small number of cases that are not offensive. In those cases, the celebrities in question would likely not want to endorse those products anyway, and would need to have blackmailer mentalities, like I said.

Still, a fascinating creative thinking challenge. If I think of a way to make it work, I'll post it here.