I've actually been in court for this kind of stuff. (Not as a defendant.) What actually happens is the prosecutor tells the court that they have X, Y, and Z evidence, which indicates the existence of W evidence, even though they can't actually get hold of W evidence. Then they don't charge you for W evidence directly, they just run the charges up for X, Y, and Z until you agree to a plea bargain anyway.
Your ability to defend against tactics like this is 100% dependent on the size of your bank account, which, by the way, may also be compromised if the prosecutors can convince the right people that you made any money at all on your illegal activities.
This is why I am mostly blasé about things like TrueCrypt's capabilities or this particular part of the law or what-have-you. If you're doing something illegal involving a computer, you're already screwed anyway -- unless you're independently wealthy and have some serious connections.