I use a custom made http application server so I can't post exactly
the code I use but here are the "responses" to the browser:
Simply replacing http://<%cHost%>/overlay should work. I use
application/vnd.mozilla.xul+xml mime type for demo3 and text/xml for
overlay.
I've tested it with Fedora Core 7 using FireFox 2.0.0.5 and with a
Windows XP Home notebook using FireFox 2.0.0.6.

best regards,
Lorenzo Fiorini

demo3 remote procedure:

<?xml version="1.0"?>

      <?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

      <window id="demo3" title="Demo3" sizemode="maximized"
        xmlns:html="http://www.w3.org/1999/xhtml"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/
there.is.only.xul">
        <button id="loadovlbutton" label="LoadOvl"/>
        <box id="overlay" />
         <script>
         function buttonLoadOvlPressed(event)
         {
            // Load an overlay file
            var callback = {
                observe : function (subject, topic, data) {
                    if (topic == 'xul-overlay-merged') {
                        alert("Overlay loaded successfully");

document.getElementById("clickbutton").addEventListener('command',
buttonClickPressed, true);
                    }
                }
            };
            // Load the current file as an overlay
            window.document.loadOverlay("http://<%cHost%>/overlay",
callback);
         }
         function buttonClickPressed(event)
         {
            alert("clicked!");
         }

document.getElementById("loadovlbutton").addEventListener('command',
buttonLoadOvlPressed, true);
         </script>
      </window>

calling http://<%cHost%>/overlay this code is sent:

<?xml version="1.0"?>

<overlay id="toverlay"
         xmlns="http://www.mozilla.org/keymaster/gatekeeper/
there.is.only.xul">

              <box id="overlay">
               <tabbox id="Tab1" selectedIndex="2">
                 <tabs id="Tab1Tabs">
                   <tab label="A First tab"/>
                   <tab label="Second tab"/>
                   <tab label="Another tab"/>
                   <tab label="Last tab"/>
                 </tabs>
                 <tabpanels id="Tab1Panels">
                   <tabpanel><!-- tabpanel First elements go here --></
tabpanel>
                   <tabpanel><!-- tabpanel Second elements go here --