After looking around a little, I did noticed that some of the other Java EE implementations do seem to be of the opinion that an @DataSourceDefinition should be supported in persistence.xml.
For example, there were bugs that prevented this to work correctly in both Resin and GlassFish, and both were fixed:
See: http://bugs.caucho.com/view.php?id=4488 and http://java.net/jira/browse/GLASSFISH-14435
What is the current stance of JBoss on the interpretation of the spec?