Don't know that JRUnit would be the best tool to for testing clients calling on an application server. However, this could be done if write your own org.jboss.jrunit.extensions.ServerTestCase and withint setUp() method call main() method of org.jboss.Main. Of course will have to make sure have all the jars you need for jboss server on classpath.
Do you know what's other tool can be used for integration tests ? I have try to investigate several of them but I did not succeed to find one which fit my requirements.