I have just encountered the same problem. I even stepped through the JBoss and Tomcat code and it doesn't look like DefaultContext is honored at all if EmbeddedCatalinaService41 is used. Just in case, I'm deploying my app in an .ear.
As far as I can tell, the StandardManager is created in org.apache.catalina.startup.Embedded.createContext no matter what. Not sure where DefaultContext should be considered, maybe in Tomcats Embedded or in org.jboss.web.catalina.EmbeddedCatalina?
Am I missing something?