That would be a very questionable practice. While business owners have plenty of latitude to install security cameras, typically they may not do so in areas where individuals clearly have an "expectation of privacy" and a toilet stall is certainly a primary example of that. I'm not an attorney and I'm not in CA so I can't give you a definitive answer but there may be a State statute and almost certainly case law on this subject.
You might start by calling the gov't agency in CA that deals with restaurants and taverns (licensing, inspections, etc.) and see what they say and whether they can steer you in the right direction to get an answer.