1) Download the JBoss AS 4.0.4GA source
2) Compile the source with JDK 5.0 (result is two jboss homes, one specific to EJB 3)
3) Install the JBoss Portal 2.4.0CR3 into the default configuration for the EJB3 jboss home (into ./build/output/jboss-4.0.4.GA-ejb3/server/default)
I tried this, the portal came up just fine. I have not attempted to access EJBs from the portal, though, but I don't see why you would not be able to.
You can build JBoss AS from source. That way you get 2 versions in the output directory - One with ejb3 and the other without.
I have had good luck that way.
Thanks for the quick replies. That's I will try.