Have you tried using the new maps visualization available in Splunk 6? While it does not yet support the add as dashboard panel workflow that you are asking for, it will allow you to add a map to your existing dashboard via the XML editor.
The key issue here is that the google maps visualization is not compatible with your simple xml dashboard. You would need to convert this dashboard to advanced xml in order to incorporate this google maps panel, however in doing so, you would then lose several things, including the viz editor, drag-and-drop layout editor, PDF generation, and more. This is why I would recommend using the integrated maps now available in Splunk 6, which is compatible with the dashboard you created.
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