You can customize the wp-login and register page easily. But if you want to add meta tags, custom CSS, or javascript on the wp-login.php file you can use WordPress admin scripts. You can do this simply from Code Snippet plugin.
You can use the login_enqueue_scripts hook to insert meta tags, style tag and script tag in the head tag of wp-login.php
For register page wp-login.php the same hook can be used with URL conditions. Register page use the same hook but with action=register in URL.
Code for login page with register form only
Code for wp-login.php
