I've had numerous people ask me how to advertise on bitcointalk at all in the first place. Four clicks are required before a user finds themselves on the topic of the current ad slot auction, and that's if they know where to look in the first place.
The link right there under the banner, which is a logical place for it - "advertise here". So one click away if you're already in a thread, or two clicks if you're on the main page and need to click a thread first.
But whether the forum could use more banners is another story... I would imagine more users would start using the "Disable ads" option so it might defeat the purpose.