OK, this is a little tricky, but not too much. The trick is to use the mask ink effect and move the mask around. Immediately after your foreground pict in the cast, put a 1-bit picture castmember consisting of a huge black rectangle with a white circle the size of the hole you want in the middle of it. The rectangle needs to be large enough that its edges don't start to clip the foreground pict when you move the hole all the way to the edge.
Life is hard.