I am also interested in how to do this. I don't think that it is possible to deploy without the .war extension.
Our application requires that the root context be ROOT, rather than ROOT.war becuase of backwards compatibility issues with other applications that may have already been installed. We are migrating from stand-alone Tomcat 4 to embedded Tomcat in JBoss 3.2.7.
For the life of me, I have not been able to figure this out and I am about ready to give up. I have even tried extending the TomcatDeployer to accept "ROOT" as a potential deployment, but that has not worked.
If anyone knows how to make this work with AS 3.2.7, please me know!