I’ve looked into manufacturing IR opaque but visible light transparent plate covers. It’s possible, retail would be around $45. Wasn’t sure if I’d make my money back on the production run, or if there would be legal issues, so I didn’t continue the project.

> if there would be legal issues

In my state, this would be illegal without a doubt. Even fully transparent license plate covers are illegal.

A more interesting question is if it's legal to mount a really bright IR light near the plate. Bright enough to swamp out IR-sensitive cameras.

I looked into that, it won’t work. Try for yourself: Put a flashlight near your license plate and take a picture from 20 feet away. Chances are good you’ll be able to read it just fine.

I've thought of sporting bright IR LEDs that just mess up with lighting conditions without covering the plate. I'm not sure how well that could work in practice though