The HTML language permit to associate events on layers but this works only in some new generation browsers and in my opinion is unuseful. As T00N says you can use a graphical rect instead ;-)
What I had in mind was to have a pop-up menu in a hidden layer. The whole layer would become visible when an adjacent button is clicked. It would then remain visible by means of a mouseOver event anywhere on the layer, and would be hidden again onMouseOut. Something similar could be done by assigning the onMouseOver to objects within the layer, but the problem is that if the mouse hits a space between objects the layer would disappear again.