You can make the configuration changes in
jetty-plugin.sar or jbossweb.sar (depending upon version).
See here for more info on jetty packaging and config.
The AJP13 stuff only works with Jetty 4.1 - JBoss currently ships with 4.0.1 (4.1 is not considered "stable" yet).
If you want a jetty plugin that contains 4.1, let me know and I shall try to release one....