I notice your site distinguishes some Mobile21 staff who worked on this game.
I know Mobile21 was a joint Nintendo/Konami venture as confirmed by the archived website/Nintendo Online Magazine.
(The archived website is at mobile21.co.jp and the logo is still available with the Nintendo/Konami link underneath it clear, so if you don't have it (given your extensive knowledge though you probably do) it's great to grab it.)
A few people like us with programming/gaming history knowledge are working on a project both to analyse the Mobile System GB and bring it back up (this is particularly notable right now because it seems unofficially the first new online Pokémon Crystal has been made since nearly 18 years, based on reversing the Mobile Trainer, adaptor, Japanese Crystal).
Do you know more about the background story on how Mobile21 worked on the game? Were there once plans for it to support the Mobile System GB too? Thanks!
While speculating, I believe the sound engine (voice effects) might be similar to Stakomi: Star Communicator by the KCE subsidiary of Konami.