If you see the video, the initial iteration just looks like specs of mud on the plate with the letters+numbers still fully readable. His ultimate goal is a custom plate frame that looks normal to the human eye but can block camera readings.
I can see this as being legal because it's still human readable.
Plate readers aren't expected to be 100% reliable as is (angles, lighting, network goes down, etc.) Plates get dirty, sometimes rusty. Also you can't test your own plate for machine readability against all the different types of systems cops use, so how could you reasonably know the issue is on your end and how to fix it?
I can see this as being legal because it's still human readable.
Plate readers aren't expected to be 100% reliable as is (angles, lighting, network goes down, etc.) Plates get dirty, sometimes rusty. Also you can't test your own plate for machine readability against all the different types of systems cops use, so how could you reasonably know the issue is on your end and how to fix it?