Get context of overlay (get parent page information) Is it possible to get the context, like the full URL, or the containing page path, title, etc., while inside an overlay page?

For example, I have a form pop up inside an overlay, and I want to, inside that form, be able to get the context of the page that triggered the overlay popup. I can't pass any information via the URL... I'd just like to be able to do something like arg(0) for the containing page (what's faded out in the background of the overlay.

Right now, if I do arg(0), request_uri(), $_GET['q'], etc., it always returns the overlay path, like http://www.example.com/path/i/want#overlay=overlay/path

(I want the 'path/i/want' portion of the URL, somehow).
Unfortunately the PHP that's rendering the child page knows nothing about the parent page over which it's going to be placed. You could write an AJAX callback that checks window.location and reports back to the server, but that would have to run after the page was fully rendered.