To get this assertion before a jury, they would have to testify which would waive their fifth amendment rights and open them to cross examination. While it might be a good public relations move, I am no so sure about it as a legal strategy.
From what has been posted on another thread it sounds like the defense strategy is actually slightly different than ignorance of the law. Apparently the defense team will try to make the case that the defendants were ignorant about how the money they paid the "consultant" was being used. Their defense is: "We paid this guy several hundred thousand dollars to get our daughter into USC because he had a legitimate business, he seemed reputable, and he had done it many times for others. We had no idea he was using our money to bribe admissions staff at the school."