Could scraping mkv's for cover_land.(jpg/png) for thumbnails be added, and used as a higher priority than cover.(jpg/png) for thumbs... also use cover.(jpg/png) as the poster.
I am in the process of re-repping my bluray and DVD collection, and using the media cover as cover.jpg and extracting the thumb (using ffmpeg) as cover_land.jpg for the episode thumb or using fanart as the movie landscape.jpg.