There are some on more ordinary cars.
e.g. http://www.licenceplate.be/plate/hihihi & http://www.licenceplate.be/plate/hahaha (both cars belong to a couple, there was a pic of both on the facebook page)

This one is funny:
http://www.licenceplate.be/plate/te-koop
(I guess it is owned by the garage)

Or sometimes even on cars cheaper than the plate itself, e.g. :
http://www.licenceplate.be/plate/amg