Janie - Kathryn told me about this modification because my wrist won't bear the weight when I'm on the ball. So you do this:
Get on your forearms/elbows and toes, as in a plank position. Then pull your toes as far as you can up into the pike position. You need to put your feet on something that will slide across the carpet - the furniture sliders, paper plates, whatever. You won't be able to achieve a full pike like when you're on the ball of course, but trust me, it's effective.