with dhtml you can track the mouse coordinates within an object, and have a global hook for the mousedown action... so if the mouse coordinates are within a certain range you can make it perform an action, if not make it ignore and continue it's regular business...
while not exactly what you need, here is a script i wrote that handles mouse coordinates etc... with some tweaking you could make it work to your needs...
http://dev3.impactpoint.net/ipMagnify.zip
(for those wanting to use this script in a commercial project, please read the licencing information, for non-commercial use feel free to play around though)