I'm thinking of offering these sites a "shared newsletter" where we create an email list together, managed and written by me with the opt-in form hosted on your site. The site owner is entitled to half of the content space to further connect with his community and create a loyal following. He may even write the articles himself if he wishes; otherwise I write them on his behalf. And since 2 websites are running this joint effort the content value will be even better than a single website newsletter--(and yes, this is an actual newsletter and not just a weekly pitch fest)
Is this a compelling hook to you? Would you take up my offer if you didn't have a mailing list already?