Response paraphrased below... Seems like for diesels there is a little hope, but also great risk. You bring something in, paid for, then find out you can't use it. For sure a nightmare scenario. I guess worst case you have to sell your dream ride to an out of state buyer, hopefully not at a loss after all the legwork.
"CA CARB emissions requirements make registering Japan market vehicles very tricky. Gasoline engine vehicles must pass CARB, and bringing a JDM vehicle into compliance can cost upwards of US$10k. Diesel vehicles may
be exempt, but that may be up to the person at the DMV when you are trying to register the car. Before you import anything, you will want to check with your local DMV and make sure as to what their registration
So that is the word from an experienced advisor. Additionally from what I understand pre-1996 diesel may be the best bet in CA as they do not require smog check ins at all. I know on my post 1995 diesel, they make me do a 'smog check-ish deal' but don't even get it on the rollers. More a visual check. I don't even think they stick a probe up the pipe.. But you are in there every two years to possibly run into trouble.