Mask within Crop area only

When using "subject" mask, have an option to create the mask only within the crop area. I am a bird photographer and often my "subject" can be a smaller part of a larger complex scene. By cropping (or selecting somehow) a smaller more precise area, capture one would do a better job at finding and masking the proper "subject". The Crop could then be resized if desired and the appropriate subject would be masked.