You need a custom OverlayWindow:
```javascript
function ProjectionHelperOverlay(map) {
this.set_map(map);
}
ProjectionHelperOverlay.prototype = new google.maps.OverlayView();
ProjectionHelperOverlay.prototype.draw = function () {
if (!this.ready) {
this.ready = true;
google.maps.event.trigger(this, 'ready');
}
};
overlay = new ProjectionHelperOverlay(map);
```
And you need to find the markers position relative to the overlay:
```javascript
google.maps.event.addListener(marker, 'mouseover', function(event) {
var p = overlay.get_projection().fromLatLngToDivPixel(marker.get_position());
// map world relative to map container
var container = overlay.get_panes().mapPane.parentNode;
var x = p.x + parseInt(container.style.left);
var y = p.y + parseInt(container.style.top);
});
```
Code found in [this thread](http://groups.google.com/group/google-maps-js-api-v3/browse_thread/thread/f30e8dfad9cb2c9b) on Google Groups.