Unfortunately Silverlight doesn't support ISO-8951-1 encoding. One option might to manage the encoding conversion by yourself by modifying the GeoRss code provided here : http://esrisilverlight.codeplex.com/ In a first time you could force the encoding to UTF-8 and check if the result is aceptable or not.
If the <b> </b> tags are removed it appears to work.
If you could expand your encoding and allow html tags this control could be useful. At this point we are finding it quite difficult to find live real world feeds that actually work.