To initialise the initial contents of a newly created GMF diagram, simply modify the code in XXXDiagramEditorUtil.createInitialModel():

private static InternetApplication createInitialModel() {
  // default code
  InternetApplication app = ModelFactory.eINSTANCE.createInternetApplication();
  // custom code starts here
  Page page = VisualFactory.eINSTANCE.createPage();
  return app;

You don’t have to wrap this logic up within any TransactionalEditingDomains, so you can use EMF directly. This can also be implemented as a dynamic template.