There is an emergency procedure in the 350 FM called Main Servo Valve Seizure - it has no symptoms or indications as to what the pilot will see if there is such a seizure. It is interesting to note that the immediate action is to shut off the hydraulics...
There is also a very old (1990 era) service letter that talks about this in an oblique way - mentions different extend retract times for the servos. Worth looking at if you're an AS350 driver...