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();
      config.facesProjectStage(stage);
      srv.saveConfig(config);
      ShellMessages.success(out, "Faces PROJECT_STAGE updated to: " + stage);
   }
}