Are there any annotators or Named Entity Recognition for license plate numbers?
There are a lot of pretty decent tools out there for text annotation in general, and given the broad nature of the task you're approaching (license plates are about as general as words), the annotation tools you are looking at should probably come from the more classical tools for annotation.

There was actually a pretty good discussion about annotation tools on this question, which should actually apply to this problem. The most relied-upon thing in annotation right now is probably brat. You can learn more about brat here.

Hope that helps! Let me know if you've got any more questions.