form field not rendering
Encountered a similar problem when trying to render the Forgot Password form in a block. 

Since the form builder for the form is in user.pages.inc, the solution is to load the include file at the same time you do the drupal_get_form() to retrieve the form. e.g.

module_load_include('inc', 'user', 'user.pages');

If you are calling the form via a hook_menu implementation, you will need to use the 'file' and 'file path properties'.