If you are that worried about cookies, for your relief, cookie based solutions do not work on e-mail ads.
One problem is that many e-mail programs do not support cookies at all or have it disabled by default.
Even if cookies are accepted by e-mail programs, another problem, is that many users use e-mail programs that do not share cookies with the default browser, i.e. the cookie is accepted by the e-mail program but there is no way to pass it to the browser because the programs are not even aware of each other existence. So, the ad server does not have a reliable way of knowing which banner was displayed to figure which banner was clicked.
Therefore I think the most viable solution is for publishers to fetch the ads HTML code before the newsletter is delivered, passing the newsletter HTML content so Google can analyse its context.