In an effort to locate the covers for a few of the missing Japanese GBC games, I discovered that they are, I believe, included as part of a GB memory cart package released only in Japan.
http://todays-game.seesaa.net/article/283425957.html
The list also include the game 'Koguru Guruguru: Guruguru to Nakayoshi' in which I also couldn't locate a cover art on Japanese websites. Anyway I can only barely figure this out.