A goofy module that allows showing one hierarchical vocabulary in the selection form widget and in the display formatter while linking the displayed value to the entity reference filtering links of a separate non-hierarchical vocabulary that the hierarchical vocabulary is connected to by reference fields on its terms.
Currently existing as a proof of concept with hard-coded field names:
field_filter_locationfield_alternate_name_if_parent
With that latter a good idea for the field the on taxonomy terms which will replace the label if this field is filled in and the term is serving as a parent.
This alternate label can be used to hide a parent level of the hierarchy by being set to the text ""
The former field machine name really has to be made configurable for this module to be useful. If you want it, please file an issue https://www.drupal.org/project/issues/dofacshs?categories=All