Ok, the Google ad will not show on the home page or category page, only on topic pages. Scripts from another ad provider does not work and the side will not load. The HTML widget, me thinks, should be able to render any HTML/JS script.
Initially, we kicked around the idea of supporting multiple categories per topic (so, more like a "tagging" approach, as opposed to categories).
In the end, we felt that having a "middle-ground" between statically defined categories and user-defined tags was not a proper compromise (since it ended up not being greater than the sum of its parts), so we stuck with static categories.