You could try to make the center object invisible. An opacity mask doesn't work in populations (which is too bad), but I lately had to vary a population of people wearing helmets, where some shouldn't wear one, so I merged a glass shader with the normal shaders, and had it controlled by simple shapes. So where the simple shapes were, helmets would be transparent/invisible.
So in all pops you should merge thus by a simple shape at 0/0/0, and see what shows.