If a website that is not primarily engaged in massive copyright violation posts a link to a questionably copywritten work, then the onus is on the rights holder to notify.

It would seem like cases where the rights holder is unknown or has lost interest in the work (aka ophaned works) should be tailor made for this. HN has safe harbor, the rights holder has the possibility of protecting their work if they choose, and the public has access to a work that would otherwise be buried in a strictly pre-legally cleared system.