Locking a servo based gimbal

I have my SplashDrone from my Avatar running on ArduCopter (meanwhile new version 4.5). A water-proofed Caddx Vista is mounted on a two-axis servo gimbal.
In some situations I would prefer locking the gimbal in its actual position, so that it is coupled to the copter’s attitude instead of following the horizon. Ideally I would like switching to this mode via an aux channel.
Is this possible?