I have created a replacement on the 2.1.8 branch that is due to be committed on the Struts 2.2.0 release down the road.  The plugin works well with the 2.1.8 source code and lets you use the Freemarker filter in web.xml like you did before without problems:

    <filter>
        <filter-name>sitemesh</filter-name>
        <filter-class>org.apache.struts2.sitemesh.FreeMarkerPageFilter</filter-class>
    </filter>

The plugin is marked as a 2.2.0 snapshot, but is built on the 2.1.8 source.

 Download it here: struts2-sitemesh-plugin-2.2.0-SNAPSHOT.jar