AFAIK, you can delete the ROOT.war under the tomcat service archive directory and create your own ROOT.war file in the deploy directory. Then, you can configure the URLDeploymentScanner in the:
and add the external path (/opt/rootcontext/) to the directories that the deployment scanner will scan. Then you would drop your ROOT.war file in the /opt/rootcontext/ directory.
So theoretically, yes, you can create a ROOT.war file in a directory external to JBoss. But I've never tried it myself, so you'll have to verify. Let us know if it works.
If it doesn't work, try doing the same thing by overriding the context setting in the jboss-web.xml file (rather than naming your application ROOT.war).