I concur with Kona on posting to the developers group.
That said there are a couple ways you could do it, I assume they'll say something similar. The canvas admin would use the custom javascript file to inject the content into the page. Most likely prepending to the element <div id="content" role="main" >.
If the content needed to change on a regular basis, it could be an iFrame to a simple html file you edit or more complex it could pull from atom/rss and update whenever something new is posted to say a category on your website.
If it needed to target a specific user group the javascript would have to figure out what group the user belongs to. At the bottom of each page there is a chunk of javascript, in that -- current_user_roles":["user","student","teacher","admin"].
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