UPD: Proper Japanese number plate (33×16.5 cm): download.
Since 1.3.4 (or whatever version it was) some cars get number plate, which is nice detail.
There are .max (2013), .fbx and some textures for KN5 (don’t forget to take skin textures from, for example, M3 E92). I think it would be pretty useful if all number plates will look the same and use same texture names.
Here is a PSD-file, which allows to create custom Kunos-style number plates (of course, with normal maps).
These number plates are better and could be created much faster.