So, the roads up to Mt. Chual and Loma Prieta are tres suck. Not only are they mostly dirt roads once you get close to the summit, but they run up the back of the mountain up to locked gates, so even if you stop short of the summit, you have no exposure to the valley.
Driving along Skyline from highway 17 to Page Mill Road is no better. But it looks like there is a road running up to the summit of Black Mountain. The Longley-Rice analysis for 50 watts ERP of 423 MHz from up there looks pretty decent. Maybe that will be a good place for the test transmission(s).