Do companies exist that represent clients (publishers) to advertising agencies or directly to firms with products matching a publishers niche? I'm thinking along the lines of a company securing 100% of impressions for an ad space (or multiple spaces) on a website and then representing that website to their advertising clients.
I realize Burst/Tribal/Adsense all work in pretty much the same way where they're an "agent" selling advertising on your site, but the agreements are informal and these "agents" represent thousands of generic publishers.
I'm thinking more along the lines of talent agents, etc, that see your site as one with potential and have the connections to secure quality advertisements for their alloted ad spaces.
There is a similar topic in the Supporters Forum right now. So far nothing has jumped out to make this an easy find. Direct sales, in my opinion easily perform better financially than the networks like you've mentioned. The challenge as a website owner is do you add content and keep things running, or spend time doing direct sales?
There may be a niche market out there to earn significant money representing a few websites.