Just a thought. This would be basically your all around dummy object, except it would have no image attached. Instead you would attach it to another object, much like the drop shadow object works.
Once it was attached it would then copy it's host's image data with out color for per pixel collisions, in fact it would still have zero opacity, but could be used as a collision mask. I think to do this though you would need an object as well as a behavior, so that you could attach a sprite to it, instead of the other way around.