Interesting idea.
I am sure you can make a circular/oval frame, and even mask the inside, but it would depend on the character you use. MSagents or Guile's, or Some of the others, something NOT Haptek can be sized and positioned anyhere, but i never saw a haptek without a rectangle background.
It could be possible to edit the backgrounds to fit your opening, then it would "Look" round even if the player positioning was a square area.
Im thinking outloud, but im sure it would be possible.
Some of the Haptek/Putty people on here will know for sure.
I like the idea.