Rainbow is a project that offers documentation for various protocols and mappers. The Rainbow Net protocol documentation is available, along with the NES Rainbow mapper documentation for the NES Mega Drive/Genesis (work in progress). The Mega Drive/Genesis Rainbow mapper documentation is also being worked on. If you have any feedback or would like to create your own ROM and server, feel free to reach out. The developer, Antoine GOHIN of Broke Studio, is open to testing on real hardware. Contact information includes email, a contact form, and various social media accounts. Joining their Discord server allows for discussion about the Rainbow project and more.
https://github.com/BrokeStudio/rainbow-net