Other than a few coastal places like Marin and Sonoma that aren't on the state water project I think the answer is no. The resulting water will be too expensive. It would be cheaper to pay farmers to stop growing inefficient and largely unnecessary crops (pasture grass, alfalfa) or to move to more efficient drip irrigation. As you said, the domestic use is so low and ag users wouldn't pay for desalination when there is actually a fairly sizable (although possibly dwindling) source of water available.
Most coastal communities in southern California don't even bother to reservoir off their water during rain events, instead letting it drain into the ocean. One major exception being Santa Monica which only uses 40% of it's water from up north unlike other communities which are 100% reliant on it.
Back of the envelope math suggests that based on average rainfall amounts per year a place like Ventura county could be entirely self-sufficient without any desalination plants.
Agriculture can also switch to drip feeding water instead of spraying which cuts usage by 90% without lowering yields.
There's a lot that can be done. We just need the political will to make it happen.
Having extra reservoirs can also support more natural vegetation and potentially actually create conditions for more rainfall further from the coast.
You could get a lot of mileage out of just cutting out that billionaire's almond groves. Or make him pay for his water; at last report, he was stealing it with a pipe stuck through the side of a canal dike.
