The URL for the seedlist needs to be pre-fixes with the portal context root (/wps by default) in WP8 otherwise the connection will fail with the error: "SRVE0255E: A WebGroup/Virtual Host to handle /seedlist/server has not been defined".
Therefore the correct URL should be: http://server_name:port_number/wps/seedlist/server?Action=GetDocuments&Format=ATOM&Locale=en_US&Range=100&Source=com.ibm.lotus.search.plugins.seedlist.retriever.jcr.JCRRetrieverFactory&Start=0&SeedlistId=1@OOTB_CRAWLER1
Therefore the correct URL should be: http://server_name:port_number/wps/seedlist/server?Action=GetDocuments&Format=ATOM&Locale=en_US&Range=100&Source=com.ibm.lotus.search.plugins.seedlist.retriever.jcr.JCRRetrieverFactory&Start=0&SeedlistId=1@OOTB_CRAWLER1