Drupal 7 node access per entity using a field dropdown, callback functions and the node access API.

drupal7-field-node-access.php

<?php
/**
 * Shows how to create a field that controls node access in Drupal 7 based
 * on callback functions using the node access API and a field select list;
 * so that content managers can easily control per-entity access control
 * using a simply dropdown.
 *
 * 1. Create a custom module and insert the following 3 functions.
 * 2. Create a List(text) field called field_access with widget select and
 * attach it to the entity where you want to control access.
 * 3. The values of of field_access should be a list of function|Option, e.g.,
 *
 *   TRUE|Public
 *   user_is_logged_in|Auth. User
 *
 * 4. Each function will receive the same arguments as hook_node_grants:
 *   $account and $op
 *
 * 5. The function should return TRUE if access should be granted to user.
 * 6. Rebuild access permission if needed: admin/reports/status/rebuild
 */

/**
 * Implements hook_node_grants().
 */
function module_name_node_grants($account, $op) {
  $grants = array();
  foreach (_module_name_get_functions_gids() as $callback => $gid) {
    if ($callback($account, $op)) {
      $grants['functions'][] = $gid;
    }
  }

  return $grants;
}

/**
 * Returns a map of functions to gids.
 *
 * @return array
 *   In this format function => gid.
 */
function _module_name_get_functions_gids() {
  static $gids;
  if (!isset($gids)) {
    $info = field_info_field('field_access');
    $gids = array_keys($info['settings']['allowed_values']);
    $gids = array_filter($gids, function($callback) {
      return function_exists($callback);
    });
    $gids = array_combine($gids, range(1, count($gids)));
  }

  return $gids;  
}

/**
 * Implements hook_node_access_records().
 */
function module_name_node_access_records($node) {
  if (empty($node->field_access['und'][0]['value'])) {
    return;
  }
  $function = &$node->field_access['und'][0]['value'];
  $function = $function === 'TRUE' ? TRUE : $function;
  $function = $function === 'FALSE' ? FALSE : $function;

  if ($function === TRUE) {
    return;
  }

  //first you need to negate published access in the realm all by setting
  //everything to FALSE
  $grants[] = array(
    'realm' => 'all',
    'gid' => 0,
    'grant_view' => 0,
    'grant_update' => 0,
    'grant_delete' => 0,
    'priority' => $priority,
  );

  $gids = _module_name_get_functions_gids();
  if (is_string($function) && isset($gids[$function])) {
    $grants[] = array(
      'realm' => 'functions',
      'gid' => $gids[$function],
      'grant_view' => 1,
      'grant_update' => 0,
      'grant_delete' => 0,
      'priority' => $priority,
    );       
  }

  return $grants;
}