I don't know that it's possible on recent Apple hardware. I remember reading somewhere that the green LED is triggered by the camera power line, or something along those lines.
Note to anyone developing a new webcam: if you want to be able to flash your LED to indicate something to the user, add another color, and keep the main LED tied to the power line (ideally with a hardware-implemented delayed shutoff on the power so a single-frame grab lights the LED for a long time).