Limit taxonomy terms display on hierarchical select
Workbench Access module might be a great solution for your issue.


  Workbench Access creates editorial access controls based on hierarchies. It is an extensible system that supports structures created by other Drupal modules.
  
  The module supports Taxonomy and Menu modules for the management of access hierarchies.


Also, take a look at the current issue for Hierarchical Select module integration with Workbench Access module