They also threw something like that in the (sprawling) rail plan. It was alongside numerous other interesting, but not necessarily feasible in the short term, proposals (additional service to Redding, etc.). It wouldn't be hard to envision this working out well as a medium-term measure, and depending on travel times and improvements I could see it serving a role not unlike that of the old Tokyo-Osaka overnight train (since presumably the first CAHSR train(s) won't arrive at the "other end" before...either 0800 or 0900, depending on when service starts...and won't depart after 2000/2100, depending on when the plan would be to wrap service).
On the Spirit of California, the biggest issue IIRC is that the train ran with too much coach space and not enough sleeper space for its mission (as a quasi-dedicated overnight train).