Is displaying a working Field Widget Form on its own possible?
VBO does something like this in modify.action.inc:  

$form_key = 'bundle_' . $bundle_name;
$form[$form_key] = array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; $label,
  '#parents' =&gt; array($form_key),
);  
field_attach_form($context['entity_type'], $entity, $form[$form_key], $form_state, LANGUAGE_NONE);  


So, you need the entity type, the entity (which can be a blank object with just the bundle key set, that's what's actually used), the form where the widgets are added, and the language.
If you want to embed the widgets deeper into the form (not in $form, but in $form[$form_key] as I did, or even deeper), then that form array needs to have #parents set.

Of course, notice that this will embed the widgets of all fields belonging to that entity type &amp; bundle. This is how the attach functions were written.
Going around that would require you to reinvent quite a lot of code; see the actual code that does the heavy lifting.
What I do is go through field instances, get each $field_name, and if that field type doesn't interest me, I set $form[$form_key][$field_name]['#access'] = FALSE;
which hides those widgets from sight.

EDIT: Okay, ctools has ctools_field_invoke_field() which could in theory allow you to work on a per-field basis. I've never used it though. The text above is my direct experience.