Not sure whether the reflectiveness is a legal requirement, or just an artifact of that design. For example, most plates are made of metal, that doesn't mean a sticker plate must be made of metal. The metal plates are heavier than vinyl, but it doesn't mean you must make a heavy sticker. So you don't have to duplicate every single property. If most plates are reflective, it does not automatically mean all plates must be reflective. Actually I've seen non-reflective vanity plates in my state, and I'm sure the electronic plates now legal in CA are also not as reflective.Quite a few states have reflective plates. The standard CA plate is reflective. I would assume you can make a reflective stick on plate.
PS> I've asked a couple of cops about the vinyl plates I have been using, none of them had any problems with them. One mentioned it's legal to hand draw plate number on a car cover, so why not a sticker. The other mentioned that plate covers (which may reduce reflectivity) are legal, as long as the plate is clearly visible.