gistfile1.txt

<?php

// script on registration page only
// limited testing

// can go in path/to/wp/wp-content/mu-plugins/

add_action( 'login_enqueue_scripts', function() {

	if ( isset( $_REQUEST['action'] ) && 'register' == $_REQUEST['action'] ) {
		// script here
		// echo '<script...></script>';
		// or
		// wp_enqueue_script( 'script-handle', 'script-url.js' );
		// see http://codex.wordpress.org/Function_Reference/wp_enqueue_script for params
	}

} );

or without the closure...

add_action( 'login_enqueue_scripts', 'yourprefix_login_enqueue_scripts' );

function yourprefix_login_enqueue_scripts() {

    if ( isset( $_REQUEST['action'] ) && 'register' == $_REQUEST['action'] ) {
        // script here
    }

}