Is there a reason hook_field_update doesn't update the taxonomy index for unpublished entities? I have a use case where editors can manipulate the taxonomy terms of unpublished nodes but these updates aren't inserted into the taxonomy index table. This makes it a challenge to create a view that searches unpublished nodes by taxonomy term name (as opposed to tid, which can be search by taxonomy_field_name_instance)
This would be trivial to patch as it is just an if statement in line 501 of field.api.php.