I found the following in a forum without any attribution to the original source - so it isn't the best evidence. Still, on the face of it, it confirms that it is a physical lock. It also confirms that the tail is not held in place by the pneumatic system pressures.
As the ship is rocketing upward, the tail is held fast by a large hook that is supposed to remain engaged until the craft reaches supersonic speed, Mike Moses, Virgin Galactic vice president of operations, explained in an interview with Reuters.
At that point, the pilots release the hook, though the tail remains pinned back by aerodynamic pressures. The command to actually move the tail into descent position comes after the rocket motor burns out, near the apex of the ship's altitude. Unlocking the tail is done well before then so that if the mechanism fails, the pilots can abort the flight.
(Now in the 2nd powered flight video, the unlock command came before the engine burnt out -- but that was for a lower profile flight where burnout and the apex were closer together. The quote appears to refer to the planned full altitude flights.)