I’m old enough to have worked on CNC machine tools with hydraulic servos. In the computer, the motion axis would have a position feedback register, and the program a command register. The difference between them generated a command signal to the servo valve. But if the difference exceeded some predetermined value, a separate valve would close, locking the axis wherever it was. This prevented axis runaways due to a malfunctioning servo valve. I’d want to think that the 737 rudder servo hydraulics would have some analog to that, to prevent the rudder going hard over un-commanded.
One would think that they would create a secondary independent Rudder control system. If nothing else it could have a Hydraulic Line lock that would close 2 valves (feed and return) with a pressure vent to release the pressure on the isolated PCU Rudder valve. Then they could revert to a secondary cable control system to still allow complete control of the Rudder. I’m pretty sure it won’t be easy to move the Rudder with cable only control but maybe they could add a gearbox to amplify the backup cable system that would be better.