This defect (found here) points the way to the solution. The Tomcat instance that runs in JBoss is configured with emptySessionPath="true", rather than "false", which is the default. This can be modified in
.../deploy/jboss-web.deployer/server.xml; both the HTTP and AJP connectors have this option.
The feature itself is used to eliminate the context path (eg. "foo" in http://example.com/foo) from being included in the JSESSIONID cookie. Setting it to false will break applications that rely on cross-application authentication, which includes stuff built using some portal frameworks. It didn't negatively affect the application in question, however.
Answer author Warren-blanchet