Yes, when I said "make sure the gimbal pitch is -15 deg", I meant do it by disabling the POI pitch in the waypoint plan, just as you suggested.. I do that by designing a POI that will cause the pitch to -15 deg, and immediately thereafter, another waypoint that disables the gimbal pitch. Why -15 deg? Because that leaves a good horizon in the field of view, yet still covers a lot of ground in the frame. Works very well for a default pitch for good looking video.
Another trick to consider.... if you can predict where you'll be out of range, you can set and disable the pitch just before that point in the mission. That way you can have have your cake and eat it too, sort of. You'll have full pitch control at the beginning and end segments of the mission, and only lose pitch control for the far out portion of the mission.