Pollution Attacks, the proper name for what I said
After my previous post I got a number of comments back, one pointing me at a company (allegedly) involved in this, and the term "Pollution Attack" which describes one of the two attacks I described in the previous post.
I also discovered this article about the structure of KaZaA which I found interesting. It also mentions pollution attacks and the use of published lists of known genuine files and their content hashes. This uses the Sig2Dat tool to generate a KaZaA hash for any file.