Override entityreference autocomplete and return entityfieldquery output
EntityReference uses the ctools plugin system to define selection handlers which feed into the autocomplete options.  The interface definition is defined in entityreference/plugins/selection/abstract.inc.  In that same directory, you will see the two included handlers, Simple and Views.  Each of these is defined by two files, one for the class itself, and one with an array to register the plugin with ctools.

To provide your own selection handler, first tell ctools where to look for your plugin by implementing hook_ctools_plugin_directory.

/**
 * Implements hook_ctools_plugin_directory().
 */
function wl_event_ctools_plugin_directory($module, $plugin) {
  if ($module == 'entityreference') {
    return 'plugins/' . $plugin;
  }
}


Then create 2 files in wl_event/plugins/selection similar to those in the entityreference.  The .inc file should define the ctools plugin info, and the .class.php file should contain your plugin class.  Most likely you'll want to subclass EntityReference_SelectionHandler_Generic and just override the appropriate methods.

Lastly, make sure you add the file with your plugin class to the files[] array in your modules info file so that the autoloader can find it.