I use any paint program that will output JPG or BMP files. If I'm reading you right; you want to make something like a view port. one way to do this would be to create your scene including whatever animation you wanted and then put a "Square" primitive object between the camera and the scene. Next I would open a paint program and ensure the blank canvass was white. With in the white workarea I would draw and fill a black circle. I'd save that file and then I would set the transparency map for the "Square" to reference my newly created "JPG" file. I would also set the transparencydial to 100%. when you render you will see the affect. This is a different appraoch than was already discussed but it sounds closer to what you're looking for.