In theory that would work the same way as a red light photo camera. The tag information is only going to tell them who owns the car not who was driving it. I wouldn't worry about it. He may be gathering information for a change in the speed zone. He checks speed for a couple of days and determines that everyone goes 45 mph when the speed limit is 35 mph.
I seriously doubt you are going to receive a ticket for this. That office may have been doing one of a thousand different things, and it just looked like he was writing speeding tickets. However, if he did clock you with a laser gun, and he got a clear look at you to the point where he feels comfortable in court saying you were the one driving he could issue the citation. I seriously doubt this is going to happen. Remember, some places in Georgia still issue speeding tickets using airplanes so anything is possible.
James L. Yeargan, Jr. is licensed to practice law in the State of Georgia. All information given is based only on Georgia law, and is not directly applicable to any other jurisdictions, states, or districts. This response, or any response, is not legal advice. This response, or any response, does not create an attorney/client relationship. The response is in the form of legal education and is intended to provide general information. Any state specific concerns should be directed to an attorney who is licensed to practice law in that respective state.