Changing the Theme directory

I host 25 or so installations of WordPress on my server. I have recently started a project to run all 25 out of the same WordPress directory, so I only have to maintain a single version of the core software and plugins. I have used Ryan McDonnell’s method for setting this up and it WORKS.

I have essentially symlinked ALL of the WordPress files with the exception of:
1. .htaccess
2. wp-config
3. wp-content/themes directory

The problem I am running into is that WordPress is completely ignoring the local /themes directory and is looking to the central version for its themes.

The uploads folder can be defined inside WP to the proper server path so things still function fine there.

I have tired following the “Modifying the Wp-Config” instructions to define the WP_CONTENT_DIR to both the server path and the URL. Still, no luck. As soon as I dump a theme in the central file system, it shows up just fine in all the installs. If I dump the same theme in one of the local repositories, nothing!

Please, any advice you can give on this would be much appreciated. I am SO close to getting this right!