It seems that manual editing of supprts would be more effective on complex models if you could see the slice as in the main view. This way you can clearly see potential print failures due to floating parts of the model that are otherwise obscured by the rest of the model that rises above the failure point. Even a simple cut away view would help even if it was not exactly the same resiults as the formal slice function. So long as you could still see and place supports on the visible section of geometry below the slice plane.
Happy to elaborate and load pictures that demonstrate the issue/potential.