Sharing the wp-content folder over different Wordpress multisite installs

I have four different Wordpress multisite installs which I would like to keep separate for good reasons, but I would be happy if I could share at least some of wp-content. Is there a clean way to get this done?

I tried putting these in wp-config.php:

define('WP_CONTENT_DIR', '/home/shared_root/public_html/wp-content');
define('WP_CONTENT_URL','http://www.shared_domain.com/wp-content');

I quickly learned that sharing the uploads folder over various multisite installs will create quite a mess, so I added this:

define('UPLOADS', 'wp-content/uploads' );

Sharing plugins folders seems risky as well, however. One plugin update might change the database, and then I'll have a mess on all the other sites. I've also encountered problems with custom fonts not displaying properly (at least not in Firefox), as it seems they can't be loaded from a different domain:

https://stackoverflow.com/questions/11616306/css-font-face-absolute-url-from-external-domain-fonts-not-loading-in-firefox

So then I added this to wp-config.php as well:

define('WP_PLUGIN_DIR', $_SERVER['DOCUMENT_ROOT'] . '/wp-content/plugins' );
define('WP_PLUGIN_URL', 'http://' . $_SERVER['SERVER_NAME'] . '/wp-content/plugins' );

Which leaves me with only a shared themes folder. Unfortunately, defining the plugin directory above will break the convenient Wordpress update function (Wordpress will now think the plugins folder is external and ask for an FTP login when I hit "update"). And of course, due to the Firefox issue above, some of the themes in the shared folder will also appear broken.

Is there a better way to get this done (a "Multisite for Multisites") or should I just accept the fact that what I am trying to do creates more inconveniences than convenience? Thanks!