It is not currently possible to modify the RSS Feed inside of the VMan dashboard. You can delete the widget if you do not wish to see Thwack content on the dashboard.
in fact, it is possible to modify the feed, it just won't work in most cases because Flash (used for Virtualization Manager's UI) won't load the feeds from websites without a proper crossdomain security settings. See my post here:
us-cert.gov lacks the file altogether and msisac.cisecurity.org only allows access from their own domains, as you can see in their crossdomain.xml file - http://msisac.cisecurity.org/crossdomain.xml
Apparently, you can trick Flash by using a local proxy with a permissive crossdomain.xml file - How to bypass flash crossdomain security - Stack Overflow
I ended up using a RSS widget creator online and just embedding the html into orion. But I appreciate your response