There is a hidden game of Fantasy Zone in Arnold Palmer Golf for the Genesis. Since I've never played that game, I've never seen this secret. These instructions are from an old issue of Electronic Gaming Monthly.
Opa-Opa also makes a cameo appearance in the Sega Master System game Zillion! Of course, this relates nicely with the Zillion anime.
Sega BGM CD This is a page of Japanese BGM CDs. If you look carefully, you'll notice one of them has the music to Fantasy Zone! If anyone has such a CD or tape, please contact me!