I have a 2006 WR450 that is my primary trail bike and living in northern CA there are numerous trails not far from where I live. Having seen a few WRs with plates on them, reading forums and talking to local bike shop people, I decided to try to get a plate for my bike so I could hop on a road and get to the trails nearby. I was told to go to a certain person who was in the business of helping people register vehicles with the DMV. I contacted them and she asked me what the VIN was and said if there was a C in the 7th or 8th position it would be possible to plate. I would need a certificate of lights, proof of insurance, and the title and she would get it done. I put lights, horn and mirror on the bike, got the Baja Designs alternator mod and regulator for more watts and thought I would just go to the DMV myself. I'm glad I did. I went in and they said I would need to read the document CHP888 that outlines all the requirements for a street bike and sign a statement of facts that I read it and the bike complies. (If anyone is considering doing this on any bike I recommend getting this document first.) They also said I would need to bring the bike so they can verify the emissions sticker is on it and valid. Well... I didn't remember seeing an emissions sticker on it so I went home and lo and behold, there is one on the front frame member, in front of the engine and behind the front fender. I looked at it and it says it meets EPA and CA for OFF ROAD only. Now I'm thinking, all these other guys that have plates on their WRs must have the same sticker so I decided to give it a try. I took the bike in and the first thing he said was Sorry, it's for off road only.. I told him I've seen more than one WR with a CA plate and asked how did they do it? He says it was done incorrectly and that they are getting letters from the main office warning them to not plate bikes like this and also that they are starting to review plates that have been issued to certain bikes and are pulling them. That is right from the horses mouth, but whether they follow through is unknown.
So for me, I'm going to go with Plan B and sell the WR and get something that is factory plated, KTM, Husky or something like that I suppose. I'm a bit tired of this state and it's overkill on the smog thing. Resistance is futile, as long as you choose to live here.
Hopefully this will save someone else from all the trouble. You might get it plated, but you may lose it later because they are wising up.