Well, you're safe then because the only legal singletrack that I know of on Mt Tam is the Coastal Trail and I think you could stay out of the poison oak on it (otherwise my DH would have spent the next two weeks in horrendous pain).
But you can make a fabulous circuit around Mt Tam on the dirt roads (24 miles). I might be able to track down a jpg scan of the route we like. It is a wonderful ride.