Wow, this is exactly what happened to me when I went to register my Capricorn six months back in Cali. I had no pink slip as the boat was bought from a guy who lived in Canada but raced in the USA w/ no domestic title.
I had a bill of sale and the importers Statement of Origin but DMV would not accept this as acceptable proof of ownership. I ended up having to go to the US customs office in Los Angeles and have a secretary comb through a dozen files before she came across the original customs form for the cargo container the cat was shipped in (three years prior). I also had to call the Capricorn dealer who placed the original order with AHPC and have a faxed copy of their invoice attached with the form. 90 days later I finally received Cali tags.
By the way did I mention they had no problem taking my $500 registration fee prior to any of my searching. I'll never understand the government thought process.

Best of luck...