You could program the rules of the state channel used to provide payment exactly for the piece of the torrent provided. Like you point out, that would use a trusted third party to implement a fair exchange.
We considered doing that, but like you also point out, if you had to sign each piece of the torrent, it would slow throughput to a crawl.
We considered doing that, but like you also point out, if you had to sign each piece of the torrent, it would slow throughput to a crawl.