Provide hook to determine whether user can add term to a given vocab/parent term

Wow this works like a charm.
Thanks for the patch, I've committed it right now.

Here is a sample implementation of hook_shs_add_term_access():

<?php
/**
 * Implements hook_shs_add_term_access().
 */
function shs_access_shs_add_term_access($vid, $parent, $account) {
  if ($parent == 2 || $parent == 0) {
    return FALSE;
  }
  return TRUE;
}
?>

Something in the logic of how this is called is broken. I have a term (with tid 24) that I am trying to use to represent an "Other" option to which you can add more elements.

/**
 * Implements hook_shs_add_term_access().
 */
function custom_shs_add_term_access($vid, $tid, $account) {
  $vocab = taxonomy_vocabulary_load($vid);
  if ($vocab->machine_name == 'article_type') {
  	return $tid && $tid == variable_get('sn_article_other_term_tid', 24);
  }
}

Here are the field settings I am using. I have tried this with and without allowing the creation of new levels.

When a view the JS settings, I see that the create new option appears to be set to false, despite

shs_add_term_access()

returning TRUE when it is given the correct $tid. My working theory is that somewhere there is a caching problem or that somewhere the tid is not being passed along.

Having beaten my head against a wall for a while on this, I am documenting this here to return later.

There was a small error that generally disables creating new terms if other modules tries to control access. I've committed a fix so it works now as expected.

I just downloaded latest -dev and it worked.