Two words...DUCT TAPE;D
Actually, Oregon just revised it's car seat laws regarding children that are required to be in boosters (not full seats).
The law was changed to allow children to wear just the lap belt if there is no shoulder belt. I believe this was do to the fact that the booster is designed to elevate a child to the point that the shoulder strap fits them properly. Obviously, that is not needed with just a lap belt.
You may talk to your local police to find out if California has a similar law. In my experience, a booster seat with just a lap belt just makes it less secure.