First attempt to add readonly attribute to entity fields using after_build callback

readonly.module

<?php

  /**
  * Implements hook_form_alter().
  */
  function readonly_form_alter(&$form, &$form_state, $form_id) {
    // TODO set form using UI
    $readonly_field_form_id = 'ENTITY_NAME_node_form';
    // TODO set fields using UI
    $readonly_fields = array('FIELD_NAME_FOO','FIELD_NAME_BAR');
    // TODO get language from form, also set defaul language instead of und
    $language = 'und';
    if ($readonly_field_form_id==$form_id) {
       foreach ($readonly_fields as $readonly_field) {
        if (isset($form[$readonly_field][$language][0])) {
          $form[$readonly_field][$language][0]['#after_build'][] = 
          'readonly_after_build_element_readonly';
        }
      }
    }
  } 

  /**
  * Process after_build 
  * Make element readonly
  * #after_build callback for field elements in ENTITY_NAME_node_form.
  */
  function readonly_after_build_element_readonly($element, &$form_state) {
    //Add attributes array if not set
    if (!isset($elements['value']['#attributes'])) {
      $elements['#attributes'] = array();
    }
    $element['value']['#attributes']['readonly'] = 'readonly';
    return $element;
  }