I posted this a while ago on jetty-discuss
The etc/webdefault.xml file is now in your deploy/jbossweb/org.mortbay.jetty.jar.
You could unpack this into deploy/jbossweb, remove the jar (to avoid confusion as to which copy of the resources to use), edit the org/mortbay/jetty/servlet/webdefault.xml and touch the META-INF/jboss-service.xml. This should cause JBoss to redeploy the Jetty (jbossweb) service with the new default settings for all webapps.
Take heed of the note at the bottom of my posting.
If you are doing this because you are interested in figuring out how to precompile your JSPs, let me know and I will send you an example.