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.
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).