Splash page with text from an external source in an iRule?

If you need additional information for the page, you can retrieve it using a sideband connection. You should always make the target of the sideband a Virtual Server. The pool attached to that VS would be the server (or servers) from which you want to grab the actual content. A VS target is far more flexible and forgiving. Of course, your connection is raw TCP transport, so you manually need to iterate past the HTTP headers (I'm assuming you're using HTTP here).