The register_taxonomy function is designed only for adding taxonomies to existing post types and custom post types. It’s not something that can be expanded to users.
I have just tried a couple of plugins that work with taxonomies and neither are able to attach a taxonomy to a user.
from http://codex.wordpress.org/Function_Reference/register_taxonomy :
Usage
<?php register_taxonomy( $taxonomy, $object_type, $args ); ?>$object_type
(array/string) (required) Name of the object type for the taxonomy object. Object-types can be built-in Post Type or any Custom Post Type that may be registered.Builtin Post Types:
post
page
- attachment
- revision
- nav_menu_item.
Custom Post Types:
{custom_post_type} – Custom Post Type names must be all in lower-case and without any spaces.
null – Setting explicitly to null registers the taxonomy but doesn’t associate it with any objects, so it won’t be directly available within the Admin UI. You will need to manually register it using the ‘taxonomy’ parameter (passed through $args) when registering a custom post_type (see register_post_type()), or using register_taxonomy_for_object_type().