Well, from what I know (anyone can correct me if I'm wrong), but IR receivers look for the change from low to high before outputing a pulse. If you look inside the mouse, you will see the toothed wheels that pass between IR emitters and receivers. When you move the mouse, the wheel turns, blocking and then allowing the light to reach the receiver. This is what gives the low and high signal. Because this is a pulse type signal caused by the constant movement of that wheel, you can't really turn a joystick into a mouse control without using some other sort of mechanism. You'd have to be constantly slapping the joystick to one side to keep the mouse arrow moving.