That's a really nice idea. Immediately I was wondering how you'd deal with refeeral spam, then I noticed your 'from search engine only' bit.

That does make me think there might be a fairly easy way to spam your service though: find a search page where page of spammer ranks high, do lots of visits from botnet to page of target with referring url search engine page.

You'd think that these are 'related' pages whereas in fact no such link exists.

You'll have to guard against that somehow, check back on the search page and see if it is really related or if somebody is spoofing the referrer.