[Karellen]

So my question is: has someone found a way to search for a title from which results Extras is accessible?

There are a couple of workarounds, each requiring a Smart playlists (wiki)

Create a Video_library_tags (wiki) and add the tag Extras (or whatever you want to call it) to those movies that have Extras. Then create a smart playlist to list Tags is Extras

Or you could rename the video file from Fifth Element (1997).mkv to Fifth Element (1997).Extras.mkv and then create a smart playlist with File Contains Extras.

Tags is an easier method, but unless you preserve your library into nfo files, then you will lose this listing if you need to rebuild your library.
Renaming is more permanent.
Both still require recreating the playlist when rebuilding on a new Kodi, which should only take a few seconds.

Importantly, the Extras option is available from the Smart Playlist listing.