Actually it doesn't work.
You see, all the checks and URL's in the host-node mapped subsite are relative to itself. If you have, as an example, a page called /subsite/add-edit, the user will see the add-edit link only, but the real URL is /subsite/add-edit. The form action attribute, in that case, will have the full URL (/subsite/add-edit) and it's going to post to it. When the form is reloaded and rendered, it's going to have the add-edit link only, and the vars that were posted to /subsite/add-edit will be lost.
It also happens in the confirm-template page at acs-templating and many other places, such as register page. The first solution I thought was to parse the vars in the request, as the above patch shows, but it fails if you have a big form. So I ran out of ideas.
