+1 to every "add-on charge, etc." post.
Another problem is proving whether or not the parts in question actually ARE in non-compliance, which would take a good metallurgist and proof that the person caught intentionally violated the section to begin with. Also, since many parts (US and foreign alike) are un-marked for origin of manufacture, the chances of them even trying to bust you for it to begin with would probably be a huge waste of time and money (after all, they already know that they're sending you to jail, so why rip themselves off in the process?)
The main purpose of 922r is similar to the import laws that are made to inconvenience importers that are "making money by flooding the gun market with inexpensive cop-killer machine-gun assault battle destructive device rifles". Think about it: spending a lot of money to try and bust a guy for a charge with extreme situational plausible deniability pales in comparison to busting an entire import/manufacturing company that they could easily pile hundreds if not thousands of 922r charges against them (especially since the company would have no real plausible deniability).
IIRC, didn't Tennessee Gun Works get put under investigation for something similar?