[schapplm]

I am not sure if it is reasonable for an addon to write the nfo files in the first place. It would mean "hacking" the Kodi video database somehow.
The addon right now only gets the media information without relation to the files via the json API.

Code:

http://localhost:8080/jsonrpc?request={"jsonrpc": "2.0", "method": "VideoLibrary.GetMovies", "params": {"properties": ["title", "year", "imdbnumber", "lastplayed", "playcount"], "sort": { "order": "ascending", "method": "title" } }, "id": 1}

Code:

...,{"imdbnumber":"tt2294449","label":"22 Jump Street","lastplayed":"2014-12-13 22:46:05","movieid":945,"playcount":1,"title":"22 Jump Street","year":2014},...

It would be possible to add the property "file" and then figure out the nfo file and manually change it with this addon.
I originally designed the addon to have the scope of save/restore/synchronize the watched state of the media between the Kodi database and an addon database.

Keeping the Kodi watched state synchronized with the nfo file is a function, that is already included in Kodi itself via the export video library into single files feature, and would be some kind of redundant. For this, it would be better to have an addon like "export the video library every day into single files".

That option doesn't work for me.  The nfo that is exported isn't compatible with the media manager I use.

If the request is to write a feature, that exports the state in a format, that is specific for media companion, then I would say there should be a distinguished addon or a simple conversion script for this.