Hi @SweUSV -
Apologies - I was jumping to the idea of you manually controlling your servo via the GCS software. This is also possible to do in QGC, but definitely more of a hassle.
If you want things to happen with the winch at waypoints, automatically, than the lua script related to winch control is the best starting point for your application. This would let you have a cast sequence occur at whatever desired waypoint(s) (odds, evens, a specific #, etc.)
In either case, you can indeed connect your servo ESC to one of the available outputs of the Navigator, and configure it to be used by mavlink message or lua script servo output control.