I have also considered a rain cape, but for touring. I came to the conclusion that given that I would need and don't always want to use fenders, and that my jacket is already highly rain resistant it would not be worth it and that I would carry rain pants instead.
I am not sure that it is worth building a "rain bike" for the two or three weeks that it rains in SoCal. I do what you do... drive when it rains. If going "car free," then it might make sense. I have a buddy who has no car and relies on an E-bike to get to work. He has good rain gear, but even so he says that most of the time he takes the day off when it rains heavily.