I also tried to do the same based on the CSOM docs and was off to a slow start. While I cannot give you a definitive answer at this point (I am still researching the problem), I believe I can be helpful in sharing a debugging technique on the client-side which will bring you a list of all methods an(public and private) on the client. You can use the "debugger;" javascript keyword to trigger VS.NET debugger in IE (first make sure you go to IE Options and uncheck the "Disable Script Debugging" checkbox) or FireBug, if you have the FireBug add-on installed.

    <cc1:WebDialogWindow ID="WebDialogWindow1" runat="server" Height="300px"
        Width="400px">
        <ClientEvents Initialize="handleInit" />
        <ContentPane ContentUrl="http://www.google.com"></ContentPane>   
    </cc1:WebDialogWindow>
       
    <script type="text/javascript">

        function handleInit(sender, args)
        {
            debugger;
        }

   
    </script>

Typically, the methods with "_" prefixes are private.  You can add "sender" to your Watch Debug Window to see all available options there.

In this case, you may try experimenting with the z-index of the Html element of the window available through sender.get_element().style.zIndex... setting big zIndex for the active window may resolve the problem.