How to add fields to the WordPress permalinks page

cpt-permalinks.php

<?php
/*
Plugin Name: Custom-Post-Type Base
Plugin URI: http://pmg.co/
Description: Dynamically change your custom post type slugs
Version: n/a
Author: Christopher Davis
Author URI: http://pmg.co/people/chris
License: GPL2/Creative Commons
*/

add_action( 'load-options-permalink.php', 'wpse30021_load_permalinks' );
function wpse30021_load_permalinks()
{
	if( isset( $_POST['wpse30021_cpt_base'] ) )
	{
		update_option( 'wpse30021_cpt_base', sanitize_title_with_dashes( $_POST['wpse30021_cpt_base'] ) );
	}
	
	// Add a settings field to the permalink page
	add_settings_field( 'wpse30021_cpt_base', __( 'CPT Base' ), 'wpse30021_field_callback', 'permalink', 'optional' );
}

function wpse30021_field_callback()
{
	$value = get_option( 'wpse30021_cpt_base' );	
	echo '<input type="text" value="' . esc_attr( $value ) . '" name="wpse30021_cpt_base" id="wpse30021_cpt_base" class="regular-text" />';
}