Darth-walker, yours is a good question. And I see that rather than anyone answering the technical aspect of it, they are instead bristling at the thought of reporting anything, in the first place ! Ie.: if it's the "honor system", well, gee, then who's to say you didn't only find clad?"
To answer your question, even if you reported the bust half, and even if the policy was only for recording purposes, and they let you keep all your finds, ............ well that still gives me pause for concern. Here's why:
Let's say that everyone dutifully reported all their good old coin finds, all these last 10 or 20 yrs. there in NYC. And let's say that someone from the city compiled a tally, and determined the amount of $$ that was being spirited away by these arm-chair yahoos, from "city land" ? If info. like that got on the desk of an archie (who sees your bust half on the list, and all the others...), then ...... what do you think he's going to do? He'll make a fuss.
Hence sometimes its better that the public's perception of md'ing is that we md'rs just ply for recent change, match-box cars, etc...... There's no shortage of places where no one cares if you find clad. But "heaven help you" if you actually found something valuable.