Extending data with iron:router controllers

childController.js

ChildController = ParentController.extend({
  showItem: function () {
    /**
     * Here is the problem.
     * I want to implicitly pass the "group" to EVERY rendered route
     * Is it possible to do this?
     */
    var group = ChildController.__super__.data.call();
    this.render('itemPage', {
      data: {
        items: Items.find(),
        group: group
      }
    });
  }
});

parentController.js

var errorHandler = function (e) {
  this.render('errorPage', {
    data: e
  });
  this.stop();
};

/**
 * Extend this controller and parse the :groupSlug into a Group
 * Then pass the data down to the children controllers.
 */
ParentController = RouteController.extend({
  waitOn: function () {
    var subError = _.bind(errorHandler, this);
    return Meteor.subscribe('group', this.params.groupSlug, {
      onError: subError
    });
  },
  data: function () {
    return Groups.findOne({ slug: this.params.groupSlug });
  }
});