There might be work around to get a button to put on dashboard itself to do a export but spending effort just to do that would be futile. Rather your best bet is to create a button in the portal itself as Download Dashboard Powerpoint and then link the button to download a cached PPT export of the dashboard. This will be much easier to do and user would not even know the difference.
I however kind of liked the idea of challenging the dashboard tool's capability to the limit.