I worked with color transparency materials a number of years ago. It seems that the problem of contrast control has always been a consideration in the use of these materials, especially when one attempts to make prints from these types of materials.
If your aim is to make prints from transparencies and this is why you are trying to control contrast then the attempt to precisely do this in camera appears to be almost impossible to attain. Certainly, one can use graduated neutral density filters and they are effective in certain situations, but to precisely mask is another matter.
I masked my transparencies, when working with that material, using a low contrast and low density black and white film mask. This became the negative of my positive transparency image. As such it added density were it was of lower value in the transparency and did not add appreciable density in higher density regions on the transparency. Using this method proved to be quite effective, in my experience.
If you decide that you want to explore this method and have questions, you will probably find others who have worked with this procedure. I will also be happy to share what my limited experience has been. Good luck.