Which position should it go to? The vehicle firmware doesn’t persistently store its PWM outputs, so it needs to either choose an arbitrary value to start at (the middle seems pretty reasonable for something it thinks is a camera mount), or start without any value (which could have issues if a servo is supposed to be holding up a camera, because it won’t put energy into maintaining its current position if it’s not receiving a position target).
A couple of solutions I can think of:
ArduSub could be modified to include parameters for the default mount PWM values
You could write a program that sends a starting value once the vehicle connects
This could conceivably be dynamic, by reading the last value from the latest available log file
It may be possible to include such a program on the Onboard Computer, or build it into a control station software