I wouldn't be opposed to some sort of sponsored post system (kind of like the ads on Google search). For example, you could pay to have every n th post on the "trending" page of a tribe of your choice (cost depending on tribe popularity) be a post of your choice. I don't think it would be intrusive provided that n was large enough, although I do think these posts would have to have some sort of indicator to show that they're ads.

(Obviously there would need to be some basic restrictions on ad content to stop people putting NSFL stuff on random tribes.)