First I would like to point to you that JBoss Portal is not in active development. GateIn (http://jboss.org/gatein) is our current portal on community side.
Second when dealing with clustering issues, it's better to have the enterprise version and support contract where you are more likely to get precise help.
Now back to your quesiton: What are you trying to achieve? JBP 2.7.2 has a seprate download for clustered portal and that already uses JBoss Cache. Why would you not use that instead?