This project reminds me of the Public Light Bus in Hong Kong. It is a van with 16 seats, and they operate point to point on popular routes like Mong Kok to Sai Kung. They usually only leave when they are full or close to full. So you could have three mini buses to three areas in the Bay Area from a point in SF like SOMA.
I think if you modeled your service after the public light bus. It would be easier to start because of the lower cost to rent a van.
http://en.wikipedia.org/wiki/Public_light_bus
Good luck!