Treat WordPress custom taxonomy as meta data

add-term-if-not-exists.php

<?php
// check for a term and insert it if it doesn't exist
if (!get_term_by('slug', $term_slug, $taxonomy)) {
	wp_insert_term(__($term_name, 'localization-key'), $taxonomy, array(
		'slug' => $term_slug
	));
}

tax-box-to-meta-box-radio.js

// remove all functionality from hierarchical taxonomy UI box (other than term selection)
// *and* convert the checkboxes to radio buttons to enforce single selection (this is untested)
$('#taxonomy-' + yourTaxonomyName)
	.find('.category-tabs, div:not(".tabs-panel")').remove().end()
	.find('.tabs-panel').removeClass('tabs-panel').end()
	.find('input[type="checkbox"]').attr('type', 'radio');

tax-box-to-meta-box.js

// remove all functionality from hierarchical taxonomy UI box (other than term selection)
$('#taxonomy-' + yourTaxonomyName)
	.find('.category-tabs, div:not(".tabs-panel")').remove().end()
	.find('.tabs-panel').removeClass('tabs-panel');