ember-cli-build.js

/*jshint node:true*/
/* global require, module */
var EmberApp = require('ember-cli/lib/broccoli/ember-app');
var MergeTrees = require('broccoli-merge-trees');
var Funnel = require('broccoli-funnel');

module.exports = function(defaults) {
  var app = new EmberApp(defaults, {
  });
  var appTree = app.toTree();
  return new MergeTrees([appTree, new Funnel(appTree, {
    files: ['index.html'],
    getDestinationPath: function() {
      return '404.html';
    }
  })]);;
};

This shows how to copy index.html to 404.html after your regular app build has completed.