Reading this previous incident report at
http://www.aaib.dtlr.gov.uk/bulletin/feb01/n14065.htm , you get the impression that it's almost as if it's the B737 rudder actuator problem in reverse (i.e. instead of seizing "hard-over", suffering instead from an excess of motility)
How much does the A300-600 rudder limiter system restrict rudder travel when enabled (gear and flaps up)?
The DC-8s had about 7 degrees with gear and flaps up...15 degrees with both down.
Perhaps the rudder limiter system faulted (permitting the greater travel at 250 knots - as the autopilot coped with the wake turbulence upset) ??
It's beginning to sound to me like a hydraulic hammer may be induced by the rudder limiter-valve cycling rapidly (much like audio feedback can cause a superheterodyne squeal). When they hit the 747 wake, the Flight Control System would have made a much larger than normal rudder input to correct the yaw and perhaps set up a hydraulic reverberation in the rudder limiter valve line that caused the rudder's large lateral oscillations, thereby setting up a destructive rudder-induced flutter in the vertical fin. We've all heard the very noisy hydraulic hammer that you can get in household water-pipe plumbing. If you didn't turn the tap off quickly, you'd swear the wall was going to fall down. Due to the corrective input being from the autopilot (and not the pilot's rudder pedals) that may be a factor in the destructive hydraulic hammer being aroused between the limiter valve and rudder actuator. The biggest factor in the reinforcing (or damping) harmonic of hydraulic hammer is the distance between the two "chattering" hydraulic line components and the feedback harmonic that can be set up. Some hydraulic systems necessarily have Quincke valves incorporated - coils that are designed to soak up these types of destructive hydraulic chatters. The initiator of this rudder-induced flutter may need to be an external force (such as wake turbulence) requiring a rapid autopilot input (i.e. to say that normal rudder pedal input and pilot reaction times would not create the conditions for hydraulic chatter).