I attempted to use this functionality and failed first time round. My discovery as to why was followed by a quick forehead slap. I had attempted to use the xml validation functionality of the XML disassembler component rather than the XML validator component.
See this doc for more information on how to configured dynamic schema discovery for the xml validator. Basically leave the doc schema feild empty. http://msdn.microsoft.com/en-us/library/aa559670(v=bts.70).aspx