Bummer about the lack of "hiding" geometry, as that is usualy a "given" in these cases because of the nature of the container shape.
Not sure what to tell you other than might you have something else you can hide it with?
Or maybe use the alpha of the current circle geometry as a mask in post to clip off the corners maybe?
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com