Can the Coast Starlight route be extended?

[jruff001]

Those work in Silicon Valley tend to own properties between Orange County and San Diego area. A through train, preferably overnight sleeper train would make a lot of sense.

I am not sure that is as common as you seem to think ("tend to," like over 50% of them?). But even for those that do, while "overnight" meaning leaving in the evening and arriving the next morning might attract a few of them once in a while, no way are they going to want to be in transit for 24+ hours each way to spend a weekend at their second homes.