A register run by whom?
And of what? I don't believe that there's a serial number on any of my cheap RIFs, although the BFGs have numbers.
I do approve of you trying it at your events though, and would be interested to hear how that works out.
Only if the guns eventually make it to sites, which I'd imagine would happen at least two or three transactions away from the thieves. The whole lot have probably been traded for a day's fix, then they'll be punted on by the fence or dealer as sweeteners to their under-age runners. Most of them will languish in wardrobes, wrecked, or be chucked in a canal rather than sold on to anyone who'll have a clue what they really are.