Moving the excersise from a flat to a declined angle, simply shortens the motion of the lift. When performing close-grip benches, you should have your elbows IN, and the bar should be touching at about mid-stomach level, or an inch or 2 below where you touch with regular flats. It would be hard to keep the elbows in on that angle.
Now that Im thinking about it, it would be hard to keep the wrists locked into position too while trying these on a decline, as the weight would be rolling back into the palms, and forcing the wrists to bend back.
I don't think it is a very good idea to be frank.