I've just started looking at Document!X but I don't see howor even if I can do this. We're building an API that will be partly Advertising supported and I'm wondering if it's possible to include google ads on each (or some large subset) of help pages? In particular the Web output, but anything else would be a bonus if it works.
You could certainly do this by creating a custom template and modifying the page layout in the custom template to include the necessary Adsense script tag / html tags. This wouldn't work in CHMs or local help as they don't have a valid public Url, but for web output you would be fine.
There is a movie here describing how to create a custom template:
http://www.innovasys.com/movies/movielist.aspx?tags=creating%20a%20custom%20template
Let me know if you have any problems.
