server.xml:
      <Host name="myhost" appBase="webapps/myhost"
        unpackWARs="true" autoDeploy="true"
        xmlValidation="false" xmlNamespaceAware="false">
        <Alias>myhost.ru</Alias>
      </Host>

ROOT.war/META-INF/context.xml:
<Context docBase="${catalina.home}/webapps/myhost"
  path=""
  reloadable="true"
  unpackWAR="true">
</Context>

из build.xml:
  <target name="localDeployasROOT" depends="compile"
    description="Deploy приложения на локальную машину разработчика">
    <deploy url="${local.manager.url}" 
      username="${local.manager.username}"
      password="${local.manager.password}"
      path="/myhost/ROOT"
      war="file:${dist.home}/ROOT.war"/>
  </target>