What you'll probably want is the DOMNodeInserted event [1]. This is how
I, for example, insert favicon into Google Reader entries [2]. If your
page has some transactions "in" the page, and some added later by AJAX,
it should be a simple matter to handle each, individually, by a helper
function: grab each element in the page, when the script runs, for
processing, and then handle each new one as it is added.
[1]
http://www.w3.org/TR/DOM-Level-3-Events/events.html#event-DOMNodeInserted
[2]
http://arantius.com/misc/greasemonkey/google-reader-entry-faviconizer.user.js