CC, I was wondering about that route south of San Carlos as well. Never been to San Carlos on a bike, just in my truck on a surf trip. From what I remember, it really looked like the "end of the trail" so to speak but then again I was in surf mode and not dirt bike mode.
When I get down there to surf this winter I'll do a little exploration in the truck and ask some of the locals. One thing is for sure, if there is a way though, its certain to be pretty burly.