Re: Is there a plugin for sitewide comments?
Found multiple plugins. Most are very old, and probably wont even work anymore, but just try them.
The best I found: http://1uthavi.adadaa.com/ada-wpmu-sitewide-feed-plugin/ (makes a feed for pages and posts as well).
http://wpmudev.org/project/WPMU-Sitewide-Feed-Plugin—ITDamager (older version of the first plugin, only has comments)