AIR usually only helps the catalyst get up to operating temp. If you can block off the valve without it being visible to the smog tech he'll never know. The net result to you will be less free oxygen to the catalyst, which, with your modern fuel control will allow the catalyst to do it's job better, since it'll be controlled by the lean/rich switching performed by the computer (provided of course, that your MegaSquirt does this).

1990 Montero RS (In pieces... for now)

KG6VNX