If you are running JBoss as anything other than root, then you will not be able to bind to any port under 1024 - for security reasons. You should not run JBoss as a privileged account anyway.
Refer to this document for some details about achieving what you want. Most of it applies, although it is directed at Tomcat. The only thing you don't need to worry about is proxyPort as the URL rewriting issue does not seem to apply to Jetty. The document uses the approach of iptables for port remapping. This should get you a start for remapping and you can refer to the iptables documentation for more details.
Hope it helps.