As far as blow back valves are concerned I have never heard of them being fitted to 747 rudder hydraulics
From the Boeing maintenence training notes:
"The [rudder]Actuators are snubbed at the end of the stroke and contain Load Limiting Relief Valves set at 2800 psi."
Regarding the rudder ratio system, the actuator has an AC electric servo. If main bus power is lost, the actuator will remain in the last commanded position. If power was lost at high speed, it would be interesting to see how much control you would have at lower speeds with crosswind landings.