Ability to configure per-level if the user can create new items

I have a taxonomy vocabulary with levels continents > countries > regions.

The continents and countries are limited and complete, so no new terms should be added here.

For regions the "create new term" option is very interesting as there are too much regions to put them all in advance in a taxonomy.

Before I found the new version of hierarchical_select I have been searching for ways to make the last level a free tagging vocabulary and have hierarchy in the free tagging vocabulary, so freetagging tags have parents, but that wasn't possible. And http://drupal.org/project/taxonomy_other works only for flat vocabularies. Also http://drupal.org/project/cck_taxonomy_ssu, that promises hierarchical free tagging vocabularies was not perfect as the user interface for adding terms was not user friendly in my case.

The solution hierarchical_select provides for this problem is great, because allowing to add terms means the combination of a fixed vocabulary and a free tagging one I was searching for. Unfortunately in my case I'm missing a checkbox "only allow to create terms at the deepest level".