So I did this and it works in Firefox 3.5 on Windows XP. It shows a box with some text, an image overlay, and a transparent div above that intercepts all clicks.
<div id="menuOption" style="border:1px solid black;position:relative;width:100px;height:40px;">
sometext goes here.
<!-- Place image inside of you menu bar link -->
<img id="imgOverlay" src="w3.png" style="z-index:4;position:absolute;top:0px;left:0px;width:100px;height:40px;" \>
<!-- Your link here -->
<a href="javascript:alert('Hello!')" >
<div id="mylinkAction" style="z-index:5;position:absolute;top:0px;left:0px;width:100px;height:40px;">
</div>
</a>
</div>
What I've done:
I've crafted a div and sized it to be what a menu option could be sized to, 100x40px (an arbitrary value, but it helps with illustrating the sample).
The div has an image overlay, and a link overlay. The link contains a div sized to be the same as the 'menuOption' div. This way a user click is captured across the whole box.
You will need to provide your own image when testing. :)
Caveat:
If you expect your menu button to respond to the user interaction (for example, changing color to simulate a button), then you will need extra code attached to the javascript you will invoke on the tag, this extra code could address the 'menuOption' element through the DOM and change it's color.
Also, there is no other way I know of that you can take a click event, and have it register on an element underneath a visible page element. I've tried this as well this summer, and found no other solution but this.
Hope this helps.
PS:
The writeup on events at quirksmode went a long way to help me understand how events behave in browsers.