Let me clarify; I meant that you could send the servo control data to the Raspberry Pi using the existing Fathom X and twisted pair telemetry connection. Then, once the data is on the Raspberry Pi, you can have the Raspberry Pi handle it however you want, by controlling the PWM shield or an Arduino, for example. You could also just incorporate the data into the existing MAVLink telemetry stream, and have the Pixhawk control the servos. You don’t necessarily need to wire up more twisted pairs or use another tether interface board.
It would work, but there are some considerations:
It might be hard to fit all of the electronics if you add the PWM shield.
The PWM shield can only drive servos up to 6V.
The Pixhawk can drive servos up to 10.5V.
Why not use the servo outputs on the Pixhawk?