I suspect that as has been stated here before, the forces required to move the tail rotor blades on such a big aircraft would be too large for the pilot to manage without assistance from a hydraulic servo. I know that from personal "feet on" experience on rather smaller aircraft where the hydraulics become no longer available (as we know, many smaller types of helicopter such as the Squirrel, A109 etc can revert to manual flight control, but even then it's very hard work to control them).
What would be better would be a design arrangement that "self centres" the pitch angle of the blades to a pre-determined, neutral setting if the pilot's normal yaw control system is lost. This would allow flight to be continued, to some sort of a controlled running landing, as on other helicopters. Pilots can be trained how to handle this lesser, although still serious, type of emergency.