I'm building an extension for the search engine of playlist.com

The page that I want to add on to is their async search page
http://www.playlist.com/async/searchbeta/tracks?

however that page is never actually visited as a top-level browser
page, it's called from
http://www.playlist.com/searchbeta/tracks#

and the call to get the async web page is made using an XmlHTTPRequest
object in javascript

I've looked into extensions such as greasemonkey to help me. But
greasemonkey only allows the editing of the top-level web page.

I've also looked at HttpFox, which can grab the content of the async
page, but doesn't allow for modifying it.

Is there any way I can grab the html content internally and modify it
before returning it to the browser window?