Game.com (or as known internally "Tiger DMG") apparently uses a bog-standard SHARP microcontroller with a built-in monochrome LCD interface.
Other than a vastly different instruction set (which is undocumented btw), it's indeed basically a weird Game Boy with Game Boy Color features on a chip.
I don't think it could run at full speed on an old3DS (I'm serious), but other than the fully undocumented instruction set, everything else is documented, so it's definitely doable.
In fact, there are so many similarities with the Game Boy and Game Boy Color, that I think I can infer the cycle timings for almost all of the instructions, and possibly even the CPU bugs as well, without even owning a Game.com myself.