suppose that we have a complex web app that we don't want to modify directly (probably because it is too long to figure out, but also possibly because we don't even have the source for it). Instead, we want to add a few extra features to how some of its pages get displayed by directly manipulating the html pages that it generates. I think in many cases from programmer standpoint to write code that edits an html page matching some criteria is a whole lot easier than figuring out just how that page gets generated by the original script and altering that. So presumably our mashup would function as a proxy letting the AJAX calls to pass through to the underlying web app but serving pages with some modifications as needed. E.g. maybe it would append the words "1337 HAXORZ, W00T!" to the title of each page being served.
Incidentally, note that we will still control the server with the underlying application, if that is of any relevance to the situation.
I tried RTFA on Wikipedia and some other places about "server-side mashups" and it is as if it's written in Greek. Not just modern Greek, more like the proto Achaean... I guess the general impression I got from there was that these mashups have something to do with "web services", not with actual modification of complete html pages.
Ok, so any suggestions? Is what I am proposing here called by another term that I should google for? Are there frameworks available for doing things like that? Or is this something that actually nobody has ever been interested in doing in the first place?