as far as i know in the "mechanical backup" the rudder is still operative but i was also told that it's the only surface still connected mechanically and it should work in any case even in a complete hyd loss scenario.
The term mechanical back up refers to the loss of all flight control computers which would leave you with Stab Trim, Rudder & thrust control (symetrical giving some additional pitch control & asymetric giving additional yaw control).
With no hydraulics you only have thrust control.
As to the question of rudder control try moving the rudder while shut down on the ground. Now add the aerodymamic forces at anywhere from 0 - 350 kts
