You might try exploding the war (if it isn't already), then unjar the WEB-INF/lib/jbpm-webapp-3.1.jar file and place the classes in the WEB-INF/classes directory and remove the jar file (or change its name by giving it a .bak extension). This will give you direct access to the class files used by the webapp. Then you can make your changes to the Beans in your editor of choice and copy the resulting class files into the WEB-INF/classes directory (in the correct package directory too of course) overwriting the ones that you unjarred. This might be the easiest and most transparent way for you to make sure that your updated class files are getting used by the server. Then if you get everything working the way you want it, you can worry about rebuilding the application later.
or download the sourcecode and use the ant tasks for building the webapp