I can see that implementing this feature could be time consuming. This was just a suggestion for something I would find handy.
The problem with interpolation between layers could be a problem. But manual drawing is worse. Manual drawing often creates a mesh looking like stairs. Drawing with an intensity mask (manually between slices (or sphere brush?)) will probably create a smoother interpolation just like thresholding.
I’ve been working with models of congenital heart diseases where the CT images of the small patients often are diffuse because of radiation concerns for children. This often creates the need for manual post drawing. I also been segmenting orbita fractures where the details often must be enhanced with manual drawing (I know about this post). I often find it easier/effective to go through the segmentation slice by slice to fix it. Here is how I would use my suggested effect for orbitas (pink color to clearify):
