Hi Guardian, hmm, what would you want this for? The obvious answer is that you have a square screen (well, rectangular), and what with the old square peg into a round hole adage... The only way to have different shaped camera views (and mirror views, I should imagine, though I don't own the Pro version) is to have a panel over the top of the view with the shape you want cut out of it... And you can always use the "arc" and "aspect" parameters to get a fisheye view or a compressed view that suits the shape you have chosen...
Cheers,
Keith