I like your solution of adding the entire form and unsetting. I am attacking it from another angle-- create a throw-away temporary form and copy in only the field(s) you wish to preserve. Here's what I posted at http://api.drupal.org/api/drupal/modules%21field%21field.attach.inc/function/field_attach_form/7#comment-45908:
To add a single field off an arbitrary entity bundle (in this case, an autocomplete nodereference textfield) onto another form, create the form as a temporary form and formstate, and copy in to place that field definition. In my case, I'm working on a Commerce checkout form alter:
function example_form_commerce_checkout_form_checkout_alter(&$form, &$form_state, $form_id) {
$tmpform = array();
$tmpform_state = array();
$tmpnode = new stdClass();
$tmpnode->type = 'card';
// Create the temporary form/state by reference
field_attach_form('node', $tmpnode, $tmpform, $tmpform_state);
// Create a new fieldset on the Commerce checkout form
$form['cart_contents']['org_ref_wrap'] = array(
'#type' => 'fieldset',
'#title' => t('Support Organization'),
);
// Place a copy of the new form field within the new fieldset
$form['cart_contents']['org_ref_wrap'][] = $tmpform['field_card_organization'];
// Copy over the $form_state field element as well to avoid Undefined index notices
$form_state['field']['field_card_organization'] = $tmpform_state['field']['field_card_organization'];
..
The advantage to either solution likely depends on the complexity of the "source" form (too complex means a lot of unsets with the form-insert method) and whether the source form will ever receive new fields over time (new fields will show up on your "destination" form in the form-insert method).
Thanks for sharing your solution!