There used to be a "Ride Board" in the Student Union on the Berkeley campus - probably at CSUH, UCSF, etc. Potentially a college student is looking to get away to the mountains but can't for lack of a car. When I was active with the UC Hiking Club, there were lots of undergrads who loved to get a ride, borrow my car, or go on a trip that I'd organize/drive for. Or hang a notice at an REI store offering car use (typically they pay gas while on their own) for the period of your hike while they explore Owen's Valley, Yosemite, Tahoe, etc.
Maybe that's all done on-line now through craigslist. I can understand the desire to have someone semi-vetted through BPL or something similar, but if you require a reference from their employer and you photocopy (or snap a photo with your cell phone) of their driver's license, etc, I've felt comfortable being on either end of that sort of arrangement with strangers. Since you need to know in advance that they're on board and acceptable, I'd meet them first, exchange info, and see if they pass my "sniff test" (everyone describes themselves as a non-smoker, including smokers).