Unfortunately, I don't know of any obvious way to solve your problem. If we were talking about a pure Remoting application (i.e., direct use of Remoting, rather than indirect use by way of EJBs), I would think of some kind of asynchronous architecture. I.e., you make the invocation and get a URL back pointing to the eventual result. Now, if you want to get crazy, you could wrap the EJB3 invocation handler, org.jboss.aspects.remoting.AOPRemotingInvocationHandler (see the configuration file $JBOSS_HOME/server/$CONFIG/deploy/ejb3-connections-jboss-beans.xml), in your own handler which caches results and returns a URL. Then, when it sees the URL, it returns the result, or an indication that the result isn't ready yet. It sounds like fun, kinda.
The latest version, Remoting 3, is a completely different code base that I don't know much about.
Sorry I couldn't help.