There have been numerous prosecutions for 18 U.S.C. Section 922 (v) and/or (w). If you ask an attorney to do a simple lexis search you will see what I'm talking about. It is true that they are almost always tied to drug/gang prosecutions. For one of the more interesting cases, see:
United States v. Spinner, 332 U.S. App. D.C. 1 (1998)
BTW, that case seems to refute the assertion that it is incumbent upon the owner/possesor of the assault weapon in question to prove that it is not an illegal assault weapon.
Cheers!
edited to add: Hey this is kinda interesting. But see:
United States v. Hayes, 68 Fed. Appx. 432
Some guys in NC were selling "assault weapons" at a flea market without a FFL (i'm guessing for profit/as a business, at least in the govt's eyes). They were arrested for operating without an FFL and then their sentences were enhanced because they were dealing in assault weapons. The kicker? The weapons were probably all preban and the govt even conceded that. But for sentencing enhancement purposes, the court said it didn't matter about preban/postban status, just the features. I call BS. BTW, no drugs involved. AND, they sent a dying 80 something year old man to federal prison for nearly 4 years. Nice. Arrests occured in 2002, decision is from '03.
Do note that even this case was not a simple possession of an AWB. And if it had been, it wouldn't have even been a successful prosecution. Just an interesting (read "whacked") use of the AWB!