The best advice I can give you is to keep your cool. Specialty construction registration is only a TINY % of vehicle registrations in CA. There are about 35,000,000 registered vehicles in CA, and only about 3,000 vehicles have registered with SB100 or specialty construction.
That tiny percentage means that only a very few DMV folks have any experience with the process. It can be done, and it's lots easier if you stay focused and polite. It worked for me, even though I was refused initally. I gave the manager my cell number, and she called later in the day to invite me back to complete the process. Please keep the group informed as you go through the steps. It helps those who will use the process in the future.