I thought I saw a post here of a company making an anti-flash system for license plates but it might only work with the big bright flashes on red light cameras...

It basically blasted a lot of IR light to obscure the image whenever it sensed a flash going off.

Some of the products tested on Mythbusters were able to partially obscure the plates. The issue was that it was easy to find the full plate from a partial when you have the DMV database behind you (and a photo which gives you things like color, make and model). Seems like obscuring might be enough to make this a non-starter for private companies though.

I've heard from a person who works in the civilian section of the traffic enforcement - that while some of the products/tactics being used to obscure plates work against the automated numberplate reading software, that he's never seen one that wouldn't be obvious enough from visial instection, fail for a human operator with a photoshop-like image tweaking interface - he says they can _always_ get enough of a plate to match up with vehicle make/model/colour details.

He says the only thing he sees that "works" is flip-up plate holders on motorcycles - and that when they see them, the photos get distributed to the local police who the have a "vehicle of interest" notice for that make/model/colour bike (and jacket/helmet).

I'm not sure how kindly the law would look at you using the plate obscuring products, but these products could be a privacy measure if they are ineffective against law enforcement. The idea being once there is a human involved, the cost goes up. This would drive up the minimum price that these private companies could sell their services for.