To clarify you want to use a context that exists outside of your JBoss configuration's deploy folder? If that assumption is correct you can expand the search path of the JBoss deployer with URLs in your jboss-service.xml -- search for the string Deployment Scanning.
Say you had a war file in c:\foo called bar.war. That would be picked up if you added c:\foo to the deployment scanner URL list.
Double check the syntax of file: URLs but I think that is correct. If the war is in an ear you can map it as usual in the application.xml file.