I'm using following plugin to make my MU installation private: http://wordpress.org/plugins/more-privacy-options/
(I tried to use WPMUDEVs Multisite Privacy but was buggy with my installation, often broke login completely).
Unfortunately Global Posts Feed breaks with the plugin mentioned above. I also contacted the developer of the other plugin to find out on which side this can be fixed but maybe you could help too? Maybe we can even work out the issues with Multisite Privacy on my site.