I didn't test all the way to include an external SVG in my plugin (instead of directly dumping the content of the svg, you want to just paste the URL).
One way to do that and that I have tested, is to paste the following string :
instead of just
I have tested it on FF and Chrome, and it works.
Edit: by the way, the 'proper' way to embed it in SVG is to use the following string
"<image x='10' y='20' width='100%' height='100%' xlink:href='http://dl.dropbox.com/u/18104402/Contorno_do_mapa_do_Brasil.svg'/>"
But since browser's parsers are pretty flexible, the previous strings should work also