I'm not a whiz at this sort of thing, but an idea comes to mind...
1. render your normal screen
2. copy the rectangular portion of the screen where you will be drawing the circular image to another buffer BUT ONLY the outside 'ring' portion (i.e. what would be left if you cut a circle out of a square). That's only 14% of the square that has to be copied.
3. render to the square area with your second camera (remember to use CameraClsMode)
4. copy back to that square the outside ring that you saved earlier.
1. render your normal screen
2. copy the rectangular portion of the screen where you will be drawing the circular image to another buffer BUT ONLY the outside 'ring' portion (i.e. what would be left if you cut a circle out of a square). That's only 14% of the square that has to be copied.
3. render to the square area with your second camera (remember to use CameraClsMode)
4. copy back to that square the outside ring that you saved earlier.