It's a shame it's proprietary, but there's a neat service[1] that's been around for over a decade that reverse engineered System Link and similar protocols to make them Internet-enabled.
They don't really need to reverse engineer the underlying protocol for that service. It's enough to simply bridge the networks in a way that local broadcast still gets through.
I haven't tried this out, I wonder how well the Halo 1 net protocol works over a less reliable connection. I encounter issues as soon as I momentarily minimize the emulator and it does not recover without starting a new game.
I wonder if it is enough for XLink to simply send the packets through a networked tunnel or if it actually needs to modify the packet payload somehow. The consoles might be able to handle everything as long as they are able to communicate with each other?
I played competitive HaloCE through 3 at a professional level. XBConnect is what people playing competitive CE used prior to halo 2 and xbox live. Game was playable up to about 60ms, with <40 greatly preferred. Hosts had a large advantage, but off host was very predictable, and required leading the targets.
The same advantage existed on LAN, with competitive tournaments setup to trade host advantage for team color advantage. The last in person CE lan I attended, we were playing on modded Xboxes with a patched version of the game that allowed a third box as a neutral host, and added an on screen timer. Happy to expand on the details if anyone is interested.
It's just branded network tunneling software. They went through the effort of making friends lists and integrating IRC channel into it, but if any two people want to play each other there are a myriad of ways to tunnel local network traffic over IP. Here's one by the guy who created xemu:
[1]: https://www.teamxlink.co.uk/