I understand what you're saying. The thing is, because their dummy was being torrented by one or more people, that would really be all the evidence they need to prosecute them without the need to go overboard in breaching the person's privacy.
But again, the purpose of the dummy torrent's tracker would be to provide probable cause to further investigate, not to pin everyone on minor copyright charges. In fact, this is already the mentality used by law enforcement. Most instances I've heard of where people are caught by the government, they're told that they must delete what they downloaded and if they're caught again, they will be prosecuted.
It does sound good on paper, but in practice, it flirts with the line between privacy and invasion of that. ISPs can cooperate, but then they would break that barrier of confidentiality and privacy as well. I guess when the pieces come together, it boils down to making it impossible to monitor a person's downloads without breaking the Fourth Amendment. The only way for them to get around that is with the dummy torrent idea as stated above.
Well, the way around that is to have a law that allows it; that's the reason we're having things like SOPA, CISPA, et cetera. The law would create an exception which would need to be justifiable.
Now keep in mind this: I have not read CISPA whatsoever. Everything I know about it, which is very little, has been all here-say. Therefore, don't take what I'm saying as support for CISPA; take it as support for the method of prosecuting criminals with the methods listed above.