Display custom list of tags in post/page editor with hooks

We can override the Ajax call to wp_ajax_get_tagcloud() and do a clean hack.

<?php
/* Plugin Name: Custom Admin Tag Cloud */

add_action( 'wp_ajax_get-tagcloud', 'ajax_tag_cloud_wpse_99497', 1 );

function ajax_tag_cloud_wpse_99497() 
{
    // PASTE ALL THE MODIFIED FUNCTION HERE
    // http://core.trac.wordpress.org/browser/tags/3.5.1/wp-admin/includes/ajax-actions.php#L639
}

Restore your core files to its original state, copy the modified function inside the above container and create a plugin for it. And ready to go.