Bob Atkins , Mar 02, 2005; 02:12 p.m.
As I recall, getting Canon FD lenses to stop down or open up when removed from the camera involves pushing various pins and levers at the same time. I don't know if there's something commercially available that you can attach to the back of the lens to enable manual iris operation. You should be able to make something out of an old lens mount from a broken FD body though.
The other alternative is ito pick up any old 50mm lens that has simple iris control, like many of the old Pentax screw mount lenses. You can often find them for $10-$15 on Ebay or in used camera stores. Then you just need an FD mount to filter thread adapater so you can mount the lens on the camera ("reversing ring").