Content scripts of extensions run in their sandbox. So to intercept the request (as I understand it, you want it personalized), sent the page will not work. The only method of communication in this case — through the DOM tree, it is common.
If you need from the content script to send crossdomain AJAX request — this is done by communicating with the Background Page
, which then sends a request, gets the result and gives it to the content script via, again, of the message.