That's an interesting idea. I suppose adding the information, if known, is pretty easy. However, I think it may be a more difficult undertaking than you might think.

I know that many of us pay attention to the license plate, and add a screencap if possible. Often the plate, if valid and not a prop, can be used to help identify the car. This is especially true when using the UK DVLA or the California BAR databases.

I know that in many cases, both in the US and elsewhere, prop plates with invalid number sequences are used. This is because license plate numbers are supposed to be "cleared" if they are to appear on screen. That means the person to whom the plates are registered, must give permission for the number to appear on screen. Invalid numbers can be used without any clearance issues. In addition, fake/prop plates are often used to represent a different time period or location.

I have noticed that cetain numbers have been used on different cars in various film and television productions. I think one of the most well known is the California license plate 2GAT123. I think it would be interesting to see how often a certain plate number is used.

I have also noticed that some movies and shows set in a fictional location will used fictional license plates. "Desperate Housewives" is set in the fictional "Eagle State" and has unique fake/prop plates for the vehicles used.

Finding a way to organize all the information may prove more difficult than you might think.