A little notification does seem like the best idea to me. Obviously it would need to be zero-effort, way unobtrusive, and nice and reassuring. "Hey- on unmetered connections, this page may balance network load with your unused bandwidth. You shouldn't notice any difference in speed! [Learn More][Edgemesh CDN]" Maybe even the slightly more aggressive "Your unused bandwidth is helping to speed up other people's connections!" Pop a neat little box in the lower right hand corner on the first visit, and have it minimize/disappear after a couple seconds.

I think it's a little skeevy to have it be completely silent. That doesn't mean it has to be super loud though.