Do you really need a button? Can't you just use a sprite and test on touched or on clicked?
If you really need to make a button object invisible, place it on a layer and make the layer invisible. (I don't think button objects work when they are on an invisible layer, but sprite buttons will work when they are invisible.)
Here is an example of both:InvisibleSpriteButtons.capxkittiewan2012-06-21 14:41:17