Ok not the official reference I was looking for, but it's a start. These guys are experienced open carriers who (IIRC) cite a court case where this was established. In any case, the series of videos is interesting info for any would be 'open-carrier' in CA. There is asegment that specifically addresses how loaded mags must be carried when UOCing a firearm.
During normal locked transport of a firearm (handgun) it is not necessary to have a magazine in view.
http://www.youtube.com/view_play_lis...DFD99BEFA7E9CB