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So it seems possible, although with AdSense tracker scripts you wouldn't put the code into an onClick event. Maybe someone who actually tracks AdSense clicks successfully with Urchin can confirm it does work.
Yes, it is possible with a free page tracker that you can put on the page with the AdSense ads. It will tell you which ad was clicked when a person leaves your site through an ad click.
Sticky message me and I will tell you the name of the tracker.