How to create an index used by a WHERE extractValue(…) LIKE '…'?
A generic XMLIndex should do the trick:

CREATE INDEX t_xml_x ON t_xml(xml) INDEXTYPE IS XDB.XMLIndex;


Your explain plan will be:

SELECT STATEMENT  
 FILTER  
  TABLE ACCESS FULL T_XML
  TABLE ACCESS BY INDEX ROWID SYS15471_T_XML_X_PATH_TABLE
   INDEX RANGE SCAN SYS15471_T_XML_X_PIKEY_IX


You should be good with this, as long as your XPath expression doesn't use any of the constructs not indexed by XMLIndex.