Oops, I just wrote a #SeamForge plugin

/**
 * @author <a href="mailto:lincolnbaxter@gmail.com">Lincoln Baxter, III</a>
 */
@Alias("faces")
@RequiresFacet(FacesFacet.class)
public class FacesPlugin implements Plugin
{
   @Inject
   private Project project;

   @Command("project-stage")
   public void setProjectStage(@Option(name = "set") FacesProjectStage stage, PipeOut out)
   {
      ServletFacet srv = project.getFacet(ServletFacet.class);
      WebAppDescriptor config = srv.getConfig();
      if (stage == null)
      {
         ShellMessages.info(out, "Project stage is currently: " + config.getFacesProjectStage().getStage());
      }
      else
      {
         config.facesProjectStage(stage);
         srv.saveConfig(config);
         ShellMessages.success(out, "Faces PROJECT_STAGE updated to: " + stage.getStage());
      }
   }
}


/***********************************************************/

   ____                          _____
  / ___|  ___  __ _ _ __ ___    |  ___|__  _ __ __ _  ___
  \___ \ / _ \/ _` | '_ ` _ \   | |_ / _ \| '__/ _` |/ _ \  \\
   ___) |  __/ (_| | | | | | |  |  _| (_) | | | (_| |  __/  //
  |____/ \___|\__,_|_| |_| |_|  |_|  \___/|_|  \__, |\___|
                                                |___/

[no-project] ~/Desktop $ cd example/

[example] example $ faces project-stage
***INFO*** Project stage is currently: PRODUCTION

[example] example $ faces project-stage --set DEVELOPMENT
Wrote ~/Desktop/example/src/main/webapp/WEB-INF/web.xml
***SUCCESS*** Faces PROJECT_STAGE updated to: DEVELOPMENT

[example] example $ cat ~/Desktop/example/src/main/webapp/WEB-INF/web.xml
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="3.0" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
  <display-name>example</display-name>
  <session-config>
    <session-timeout>30</session-timeout>
  </session-config>
  <welcome-file-list>
    <welcome-file>/index.html</welcome-file>
  </welcome-file-list>
  <context-param>
    <param-name>org.metawidget.faces.component.DONT_USE_PRERENDER_VIEW_EVENT</param-name>
    <param-value>true</param-value>
  </context-param>
  <context-param>
    <param-name>javax.faces.PARTIAL_STATE_SAVING</param-name>
    <param-value>false</param-value>
  </context-param>
  <context-param>
    <param-name>javax.faces.PROJECT_STAGE</param-name>
    <param-value>Development</param-value>
  </context-param>
</web-app>