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.