Currently, JSF doesn't have any explicit support for WAI-ARIA (http://www.w3.org/WAI/intro/aria.php), except for the role attribute. <f:ajax> should be updated to use the aria-live attribute at the very least.
See EG thread here: http://java.net/projects/javaserverfaces-spec-public/lists/jsr344-experts/archive/2013-01/message/5
After looking at this in more detail, there are several attributes that one may need to apply to the element that's being updated by an Ajax:
Only the page author (and possibly the component being re-rendered) know which attributes should be set, and what the values should be. Since the regions are denoted with these attributes up-front, perhaps there's not much for <f:ajax> to do after all, because the page author can either use pass-through attributes or HTML5-friendly markup to specify these attributes on the component that needs to be updated.
I like this idea, targeting for 2.3.