In all my research, one the most interesting games I've come across is Donkey Kong 3: Dai Gyakushuu. Undumped and largely undocumented, it is need of much more research. This thread is for any new information that is found, but I've found a few things myself as well.
I recently discovered that Micom Basic Magazine ran ads for the game, as well as Mario Bros. Special. These contained beta boxart and screenshots of the games. (Starting in November, ads for Punch Ball Mario Bros. were also run, but as far as I can tell have no beta content.) In the post below I will attach images of the most noteworthy ads. More information about the games could also be on them, so if anyone is able to translate anything it could help give more information.
It also appears that the final box of Dai Gyakushuu has beta images. For reference, the box is here:
http://aucview.aucfan.com/yahoo/147387416/For further reference, the contact of Fumihiko Itagaki, the ITA on the title screen, is here:
http://itagaki.eek.jp/contact.php Even though the contact page is for some reason in English, be sure to write in Japanese. I made that mistake and thus wasn't able to get much out of him.
Finally, I also have some places to rule out, and some to keep an eye on. Trader has retro computer games, but is a bit newer than the game and doesn't go past the 8801, so don't expect to find any MZ games or anything there. A Surugaya store also stocks these types of games, but I don't know much besides they didn't have it a few weeks ago and that the prices of them were either very low or high depending on the game. Beep also had a copy, but no longer does. Not completely sure on their Saitama location, though.
I'll post any of my other finds as well.
Edit: It's not letting me upload any. It says the upload quota has been reached. Is there something I'm missing?