If a SERVOn_FUNCTION parameter is set to Disabled (0) then its corresponding output should be controllable directly 
For a standard servo-style PWM signal you should be able to use any of the outputs, but AUX 1/2/3 (Servo 9/10/11 on Navigator) are marginally easier with the currently available firmware[1], because of the existing servo_n_* joystick button functions.
That said, you can set custom functions in QGC if necessary, and more easily in Cockpit, so there are options available
There is some ongoing work to make the “servo” functions configurable to arbitrary motors, but it’s not yet merged/available ↩︎