There are an infinite set of mathematical tricks available here, some of them so obfuscated that enforcement would probably need a few years to catch up. Not to forget that printing a hard copy of the bytes of an MP3 may even warrant you free speech protection (remember the reaction to the DeCSS legal issues, like printing it in T-shirts).
This doesn't work because some of the bits the software downloads couple be "innocent" bits. E.g. the firefox installer or some random looking piece of data. Any bits can be brought into involvement by the system, hence the denial of service.
Note that due process doesn't apply to extortion. Mike Freedman gets a ton of inaccurate nastygrams from copyright enforcers, but they don't care. They keep sending them. This situation may be evil, but it exists today and it generates significant revenue. (It doesn't seem to deter much infringement, though; I guess people think it won't happen to them.) http://www.freedom-to-tinker.com/blog/mfreed/inaccurate-copy...
Also, any P2P system that downloads a significant amount of extra data won't become popular anyway, since it will be slower than non-deniable P2P. Freenet suffered from this, since all the pirates switched to the more efficient BitTorrent.
