License plates (in the US, anyhow) have a glass-bead coating that serves to reflect light back at the source. The colored or non-white lettering covers up the beads. In theory you could put a beaded layer over the lettering, which would increase the amount of light coming back from that part of the surface, but I doubt it would make that great of a difference because the reflectivity of the surface under the beads has a large impact as well.
Depends mainly on what kind of sensor is used in the camera. If they are using a visible light sensor, forget it -- anything that would hide the license plate markings from the camera would be visible to the naked eye, and a cop would be writing you a ticket for altering your plates. If the cameras are using infrared sensors, then it is possible to come up with a coating that will pass visible light, but is opaque to IR.
As far as the ad in the site, I can take a regular, undoped license plate out on a clear, sunny morning and by carefully positioning the cameraman between the plate and the sun, I can get exactly the same "wash-out" effect. Doesn't even have to be morning -- I could have an assistant bounce sunlight on the plate with a large mirror for the same effect. Too easy to fake.
Regards;
Beanbag