After finding Almost Thayer: Hardcore Hiker #12 (GC9NRJQ), dantreeman and I and decided to continue further along the trail, using our online maps as a guide for the private property line. We decided to mark this location, still within Sierra Azul Open Space, with a cache. The trail continues on.
No poison oak right at the cache currently, and the path from GC9NRJQ to this cache is smooth sailing.
Take the usual precautions re poison oak, ticks, and other hazards, and bring plenty of water. It is a long way to get here from any of the Sierra Azul trailheads.