I wonder if we could actually remove such files, even without having the "move additional files" option enabled, assuming the directory is otherwise empty.

I would either just always remove or move them, regardless of move_additional_files. On OSX I wouldn't remind removing .DS_Store files, since they only contain settings for how Finder views the folder.

I went with that approach in https://github.com/musicbrainz/picard/commit/c5e6ed201719679fde21477b87f91a53e20b00b5