Happy to do that research for $$$! If you want to do it, just go to a law library.
The two likely answers are (1) in a foreclosure the court needs to disentangle the value of the owner’s half to determine the homestead exemption (2) it just is all homestead. Courts often have to make simplicity versus fairness determinations like this.
Given that the exemption was so tiny here for so long, I doubt we have any cases on it, and California would simply follow the majority approach of other state courts.
