Gysbreght,
Three distinct dependencies-
1) Autopilot disengages when all AC power is lost
2) The flight control surfaces will (would) lock up on loss of hydraulic power (not on loss of electrical power - sorry, I left that vague). This is a different chain of events. If Hydraulic power is restored, before pressure in the lines drops, then there will be no lock up.
3) TAC drops out when neither engine is powered. Part of the race condition is that TAC would release the rudder back to a neutral (to the prior trim) position, unless the hydraulics have already locked.
Battery backup keeps the lowest level flight controls operational. But, the plane is no longer in Normal mode (Law 1), so many of the stability controls would no longer apply. That is what allows any remaining rudder to continue to turn the plane.