I'd be pretty happy with a search engine that lived on my local machine, just for pages I've already visited.

That might be a good first step towards what you're planning. Step two would then be figuring out how to do federated queries across users with similar interests.

From a privacy standpoint, I'd be less uncomfortable letting random strangers run (sanitized) queries against my browsing history (stored locally), than I would be uploading my browser history to a third party.