Yeah, I think it could work and it's a nice incentive for people to keep seeding when there's not much benefits otherwise.
Actually some of the torrent sites have rules, saying that you should seed at least 80% of your download volume. If your Seed to Leech ratio falls below the 80% mark, then you will be banned from downloading any more torrents (happened to me many times in the past). Now these sites can just give the option of making a small payment with BTC, in case the user is unable to maintain the ratio at 80%.