I think this is a good idea; there are lots of movie-making companies that reuse some plates in different movies. Could be interesting to see how many cars have the same registration, or just to see how many movies the same car has been in.

But it may not be as simple as adding a field; it should contain the country of origin as well; i mean if speaking of the Bullitt plate it should say "US, CA, JJZ 109"

I could help with this if needed.

Any ideas?