WP_CONTENT_URL

I’ve followed the instructions here for moving my wp-content folder and putting the rest of WordPress inside a wp folder, so file structure:
wp
wp-content
index.php
wp-config.php

However, I’ve got a problem with this line:
define( 'WP_CONTENT_URL', 'http://example/blog/wp-content' );

If I move my WordPress from that URL, I have to manually update whatever file I put this in. Now, I could use something like:
define('WP_CONTENT_URL', get_option('siteurl').'/wp-content');

But that won’t work in wp-config.php, which seems like the logical place to put these defines. And even so, that particular variable goes to /wp with my structure, which isn’t correct, so I would end up using define('WP_CONTENT_URL', get_option('home').'/wp-content'); or define('WP_CONTENT_URL', get_option('siteurl').'/../wp-content');, if get_option() worked at that time, that is.

This has only become an issue when using plugins_url or content_url, so my current solution is to filter both of those and not define a new WP_CONTENT_URL at all. So it defaults to define( 'WP_CONTENT_URL', get_option('siteurl') . '/wp-content'); and in the filter I can do a quick string replace to add /../. This seems cumbersome and I can’t possibly have a filter for every WP function that will use that constant. Any alternatives?