From core:
function post_tags_meta_box($post, $box) {
$defaults = array('taxonomy' => 'post_tag');
if ( !isset($box['args']) || !is_array($box['args']) )
$args = array();
else
$args = $box['args'];
extract( wp_parse_args($args, $defaults), EXTR_SKIP );
$tax_name = esc_attr($taxonomy);
$taxonomy = get_taxonomy($taxonomy);
$disabled = !current_user_can($taxonomy->cap->assign_terms) ? 'disabled="disabled"' : '';
$comma = _x( ',', 'tag delimiter' );
?>
<div class="tagsdiv" id="<?php echo $tax_name; ?>">
<div class="jaxtag">
<div class="nojs-tags hide-if-js">
<p><?php echo $taxonomy->labels->add_or_remove_items; ?></p>
As you can see, there is no hook. So the only options are JavaScript or a clone of the metabox.
Third option: Open a Trac ticket and ask for a new hook.
You could also use output buffering, but I don’t recommend that, especially on that page.