I COULD BE WRONG ABOUT THIS, YMMV
I'll add this link
http://www.defendersource.com/forum/f6/defenders-in-california-everything-you-want-to-know-59447.html
Mostly current/true explanation of the situation in CA with regards to non US spec vehicles, albeit not a very optimistic one. Bringing a car in from outside the state means a VIN verification. Not sure if the G VINs are as weird as some of the defender VINs but this is the first hurdle. If they flag your car it means no registration and they will not allow it to be titled here. Some people are more aware of defenders and what they are vs G wagens so its possible a DMV employee just wouldn't bother making a fuss.
I have heard anecdotally that most of the successful registrations of defenders here has been by using a low-key method like a DMV courier and praying nobody notices. As others have said above, this still leaves you open to being noticed later (a letter in the mail stating they know the vehicle is not CARB legal and please take it out of state, no title for you etc.). Usually if you ask CARB the answer is you will need to use a special lab/shop to make the emissions compliant, IE build a new emissions system for the car, you usually hear $15,xxx prices minimum. IMO don't ask CARB, maybe ping a local referee casually and pick their brain.
This state is horrible for cars, but it can be done. Land Rovers, Land Cruisers, G Wagens have been done and you see them driving around a lot.
IF you don't get flagged, yes the diesel exemption is to 1997. Gasoline is 1974. They do not listen to the 25 year rule RE: emissions. They will flag anything they want.