Is a San Andreas volcano possible?
Yes. In fact there are already volcanos associated with the San Andreas fault system. 

Neenach is an extinct Miocene volcano split in half by the San Andreas fault. Garry Hayes wrote a nice blog post about this volcano system: A Volcano Sundered: A Field Trip Along the San Andreas Fault.

If you'll allow some latitude in what counts as 'a San Andreas volcano', then there is active volcanism too. The Salton Buttes are a group of rhyolitic volcanos in the Brawley Seismic Zone, a set of oblique transform faults — actually in the axis of the active Gulf of California rift — on the south-east shore of the Salton Sea. You can see them in this Google Maps image (on the right; the big fields are 800 m across):



The Space Shuttle photo (left) of the Imperial Valley annotated by the USGS shows the relationship between the San Andreas fault, about 5 km from the volcanos, the Brawley Seismic Zone (BSZ), and the Imperial Fault to the south. At this scale, it's essentially one big fault system.