Probably the best approach is to create a ProxyFactoryHA mbean that will create an HA proxy for your service and bind it in JNDI. The HA proxy will include load balancing behavior, much like a proxy for a clustered EJB does. Your external client would then download the proxy from JNDI.
The forum thread at http://www.jboss.com/index.html?module=bb&op=viewtopic&t=77986 gets into this is some detail, including an as yet unresolved bug the user found (http://jira.jboss.com/jira/browse/JBAS-3194). He also describes his workaround for the bug.