You can't pass the click through by any legitimate means, and you'll run up against cross-domain problems if you tried to fake it in anyway. And I would definitely stay away from anything that looks like a clickjacking solution - it's bound to stop working (and feels evil too).
You may be able to hack something, depending on how accurate it has to be. This would involve tracking the sequence of events happening when user has put their mouse into the banner area and then left the page (inferring that they clicked on the ad). You'll miss some, and you may catch some false positives too.
It would work something like:
- Leave the covering div in place
- onMouseOver, hide the div and set an
onbeforeunload event handler that
registers a "click" through an AJAX post (or similar)
- when the mouse moves out of the banner area it means they didn't click the ad, so show the div again and remove the event handler
I'd guesstimate you'd get about 80-90% accuracy, but you're going to have to test on a lot of browsers. It's also assuming the ad loads into the same window and not a new one. If it loads into a new one then I think it's going to be even harder.