Sorry, I put this in the wrong thread. I meant to do it in the Plex discussion thread.
To answer the question, it is a server plugin, developed by me personally and it was tested pretty thoroughly over a period of a few months with a pretty large group.
The clients don't actually need to know anything about the NFO's, because they're just another output. What i mean by that is, if you look at the xbmc forum link, in the screenshots, anytime an item gets saved (for any reason), it gets pushed to all of the outputs - mb3 xml, xbmc nfo, internal database, etc. So it becomes a pretty transparent process, and more outputs can be added if want later.