I've run into the need to load a dropdown dynamically from an external system from time to time to keep data in sync. Below has been tested to work in 6.0.1:

PHP Code:
//load needed libraries
require_once('modules/ModuleBuilder/MB/ModuleBuilder.php');    
require_once(
'modules/ModuleBuilder/parsers/parser.dropdown.php');
$parser = new ParserDropDown();
$params = array();
$_REQUEST['view_package'] = 'studio'//need this in parser.dropdown.php
$params['view_package'] = 'studio';
$params['dropdown_name'] = 'test_dropdown_list'//replace with the dropdown name
$params['dropdown_lang'] = 'en_us';

//create your list...substitute with db query as needed
$drop_list[] = array('-blank-','');
$drop_list[] = array('hello','hello');
$drop_list[] = array('world','world');

$json getJSONobj();
$params['list_value'] = $json->encode$drop_list );

$parser->saveDropDown($params);