How to add custom tag among global default values?

See the code used in nodewords_basic.module. The module needs to add some functions that are called for some tasks; in particular, for the code you reported, the module should contain a function nodewords_author_form(), which should return the form fields used in the meta tags edit forms.

To make an example, this is the code that handles the canonical URL meta tag.

/**
 * Set the form fields used to implement the options for the meta tag.
 */
function nodewords_basic_canonical_form(&$form, $content, $options) {
  $form['canonical'] = array(
    '#tree' => TRUE,
  );

  $form['canonical']['value'] = array(
    '#type' => 'textfield',
    '#title' => t('Canonical URL'),
    '#description' => t('Canonical URLs are used from the search engines, and allow them to not report duplicate titles for HTML pages that are accessible from different URLs. Use a relative URL without the initial slash; canonical URLs that point to a different domain are normally not accepted.'),
    '#default_value' => empty($content['value']) ? '' : $content['value'],
    '#element_validate' => array('nodewords_basic_canonical_form_validate'),
    '#size' => 60,
    '#maxlength' => variable_get('nodewords_max_size', 350),
  );
}

/**
 * Set the meta tag content.
 */
function nodewords_basic_canonical_prepare(&$tags, $content, $options) {
  if (!empty($content['value'])) {
    $content['value'] = trim($content['value'], '/');
  }

  if (empty($content['value'])) {
    $path = '';

    switch ($options['type']) {
      case NODEWORDS_TYPE_FRONTPAGE:
        $content['value'] = '<front>';
        break;

      case NODEWORDS_TYPE_NODE:
        if (count($options['ids']) == 1) {
          $path = 'node/' . $options['ids'][0];
        }
        break;

      case NODEWORDS_TYPE_TERM:
        if (count($options['ids']) == 1) {
          $path = 'taxonomy/term/' . $options['ids'][0];
        }
        break;

      case NODEWORDS_TYPE_TRACKER:
        if (count($options['ids']) == 1) {
          if ($options['ids'][0] == -1) {
            $path = NODEWORDS_TYPE_TRACKER;
          }
          elseif (is_numeric($options['ids'][0])) {
            $path = 'user/' . $options['ids'][0] . '/track';
          }
        }
        break;

      case NODEWORDS_TYPE_USER:
        if (count($options['ids']) == 1) {
          $path = 'user/' . $options['ids'][0];
        }
        break;

      default:
        $path = $_GET['q'];
    }

    if ($path) {
      $content['value'] = drupal_get_path_alias($path);
    }
  }
  elseif (!empty($content['value']) && strpos($content['value'], '/') === 0) {
    $content['value'] = drupal_substr($content['value'], 1);
  }

  $base_url = rtrim(variable_get('nodewords_base_url', ''), '/');
  $options = array(
    'absolute' => TRUE,
    'base_url' => $base_url,
  );

  $tags['canonical'] = !empty($content['value']) ? check_url(url($content['value'], $options)) : '';
}

function nodewords_basic_canonical_form_validate($element, &$form_state) {
  $canonical_url = trim($element['#value'], '/');

  if (!empty($canonical_url) && !valid_url($canonical_url)) {
    form_error($element, t('Canonical URL must be a relative URL.'));
  }
}

function nodewords_basic_canonical_settings_form(&$form, $form_id, $options) {
  if ($form_id == 'nodewords_settings_form') {
    $form['metatags_creation']['nodewords_base_url'] = array(
      '#type' => 'textfield',
      '#title' => t('Base URL'),
      '#description' => t('Enter the base URL that will be used for canonical URLs.'),
      '#default_value' => variable_get('nodewords_base_url', ''),
      '#element_validate' => array('nodewords_basic_canonical_settings_form_validate'),
      '#size' => 60,
    );
  }
}

function nodewords_basic_canonical_settings_form_validate($element, &$form_state) {
  $base_url = rtrim($element['#value'], '/');

  if (!empty($base_url) && !valid_url($base_url, TRUE)) {
    form_error($element, t('Invalid base URL.'));
  }
}

What is the purpose of nodewords_custom.module? should everyone be able to alter it? if so we should take care to not overwrite with module updates, otherwise I'd suggest to use the module as a template, but not edit the files..

nodewords_custom.module had that purpose, in the same way tokenSTARTER.module.
As everybody can create a custom module, I removed it.

In branch 6.x-3, nodewords_custom.module will have a different purpose, and it is not supposed to be altered from the users.

Oh, probably we should document this..