Well, I followed the instructions for right-speaker removal and optical drive fan removal and still couldn't find the right screw. I couldn't hear it rattle either so figured it was wedged somewhere. Using an inspection mirror and flashlight, I finally found it. It had fallen down and wedged between the PCB and the plastic housing (below the ports). I was able to slide it toward the right until it came to the void left by the right speaker where it fell out into my hand.
Argh! I dropped my right screw into the bowels of the iMac. I can't find it anywhere and am loath to leave it rattling around as well as to tear down the machine any further.