I have written a clean-room implementation of a Gameboy Color emulator library in C++17, which I am considering embedding in my game for an easter egg. If I do this, I will be using homebrew ROMs from open source projects where I have written permission to use the ROMs, and I will give full credit in all the appropriate places. I know that I have the law on my side on this everywhere (except possibly Japan), and I'm fine with that. I have already published a game like this on the Qemu advent calendar last year, using uCity with permission. I was told that I could not use the BIOS of the system, and so I removed it from my emulator and am instead directly running the homebrew ROMs.

However, what do you think is the industry and publisher opinion on this matter? Is it better to not do this? Would it matter if there was no way to know this was a GBC game without actually going to the homebrew projects source repository? It would be no secret, but you would have to ask.

I am asking for your personal opinion on this, and if you have reasons not to, I might just scrap this. The game does not need this to ship. Thanks.

Save yourself the trouble and implement your own fantasy Gameboy look-alike. It's probably not too much of a change if you have all that in place already. Just don't use the exact design and names and you'll be fine.

Another point to consider is that having the law on your side is not the same as having legal defense. Any case thrown at you is going to cost you money one way or the other. Even a simple cease-and-desist can cost you a lot of time.

If the intent is to run games that already exist and already target actual gameboy hardware, then a fantasy console is in no way an acceptable substitute, unless OP goes through the trouble of porting the games to the fantasy console. If the intent is to run any old games, then emulating an existing fantasy console (with permission) could be an option.