I also had to include a couple of new dependencies introduced by Retrotranslator:
I had to use the 1.0-alpha-2 version of the plugin, otherwise I got an obscure java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean failure...
Unfortunately, that was not quite enough. When running the portlet, I would get an exception about a missing javax.xml.namespace.NamespaceContext class which XWork aparently needs. It appears that this class is included in the Java 5 libraries, but not with Java 1.4. I found it in the jaxp-api.jar in the Sun Java Web Services Developer Pack 2.0. I placed this jar in the %WAS_HOME%/java/jre/lib/ext folder (I tried putting it in the web application, but that didn't work... Classloading in JEE application servers has always been fun...). I also had to exclude any web application dependencies that included Xerces, Xalan or xml-apis.
No guarantees that this works for you though. But I guess as long as you don't break any of the limitations retrotranslator has, you'll be fine.