Use tag interface for hierarchical taxonomy?

Custom taxonomies are great. I registered a bunch of new taxonomies and wrote an importer to import our hierarchical taxonomy into WordPress a la XML. The problem is one taxonomy has about 1,100 terms and browsing a checklist of 1,100 things is cruel and unusual punishment.

Is there any way to have a hierarchical taxonomy but use the Tag interface (search box with auto complete) instead?

Update: this code from Bainternet’s answer gets most of the way there
(adds tag interface for the specified taxonomy, with working
autocomplete and correctly populated “most used” tag cloud), but terms
are not saved on post save. If the post had terms before, they will
be deleted on save. So I am still looking for an answer. (This same code saves terms just fine if the taxonomy is registered with hierarchichalset to false, but the point of the question is to use the tag interface on a hierarchical taxonomy.)

//remove default metabox
//change TAXONOMY_NAME to your taxonomy name
add_action( 'admin_menu' , 'remove_post_custom_fields' );
function remove_post_custom_fields() {
    remove_meta_box( 'issuediv' , 'post' , 'normal' ); 
}



//add our custom meta box
add_action( 'add_meta_boxes', 'my_add_custom_box' );

 function my_add_custom_box() {
    add_meta_box( 
//      'myplugin_sectionid',
        'tagsdiv-issue',
        __( 'New and Improved Issue Tags', 'textdomain' ),
        'tags_like_custom_tax',
        'post' 
    );
 }

 //call back function to display the metabox
 //change TAXONOMY_NAME to your taxonomy name 
 function tags_like_custom_tax(){
     $tax_name="issue";
     global $post;
     $taxonomy = get_taxonomy($tax_name);
     $disabled = !current_user_can($taxonomy->cap->assign_terms) ? 'disabled="disabled"' : '';
     ?>
     <div class="tagsdiv" id="<?php echo $tax_name; ?>">
        <div class="jaxtag">
            <div class="nojs-tags hide-if-js">
                <p><?php echo $taxonomy->labels->add_or_remove_items; ?></p>
                <textarea name="<?php echo "tax_input[$tax_name]"; ?>" rows="3" cols="20" class="the-tags" id="tax-input-<?php echo $tax_name; ?>" <?php echo $disabled; ?>><?php echo get_terms_to_edit( $post->ID, $tax_name ); // textarea_escaped by esc_attr() ?></textarea>
            </div>
            <?php if ( current_user_can($taxonomy->cap->assign_terms) ) { ?>
            <div class="ajaxtag hide-if-no-js">
                <label class="screen-reader-text" for="new-tag-<?php echo $tax_name; ?>"><?php echo $taxonomy->labels->name; ?></label>
                <div class="taghint"><?php echo $taxonomy->labels->add_new_item; ?></div>
                <p><input type="text" id="new-tag-<?php echo $tax_name; ?>" name="newtag[<?php echo $tax_name; ?>]" class="newtag form-input-tip" size="16" autocomplete="off" value="" />
                <input type="button" class="button tagadd" value="<?php esc_attr_e('Add'); ?>" tabindex="3" /></p>
            </div>
            <p class="howto"><?php echo esc_attr( $taxonomy->labels->separate_items_with_commas ); ?></p>
            <?php } ?>
        </div>
        <div class="tagchecklist"></div>
    </div>
          <?php if ( current_user_can($taxonomy->cap->assign_terms) ) { ?>
            <p class="hide-if-no-js"><a href="#titlediv" class="tagcloud-link" id="link-<?php echo $tax_name; ?>"><?php echo $taxonomy->labels->choose_from_most_used; ?></a></p>
          <?php } 
}

The original question is borrowed from the WordPress forum post here.

Here’s how I did it. Just add a conditional that checks if the page being loaded is an admin page or not. If it is an admin page, set hierarchical to false, otherwise set hierarchical to true. Like so:

$args = array( 
    'hierarchical' => true,
    'labels' => $labels,
    'show_ui' => true,
    'query_var' => true,
    'rewrite' => array( 
          'slug' => 'genre'
    ), 
 )

if( is_admin() ) {
    $args['hierarchical'] = false;
}

register_taxonomy('genre', array('book'), $args);

That should give you the idea. The downside to this is you can’t add parent relationships to terms using the admin interface. You could get more specific in the is_admin() conditional such as looking to see if the request contains post-new.php or post.php…