Huge fan of Halo 1, and for me, the original (via LAN, XLAN or Xlink Kai) is still the preferred way to play compared to Halo MCC (RIP XBconnect). I dabble in map mods so this is mostly over my head, but interesting read.
Don't know if would help your research: but are you aware of a mod/mappack called Halo 1: NHE (No Host Edition)? It's a hacky way of using a third box to host the system link game.
4 year old account, and this is your first comment? :D You might be interested in super custom xISO with NMP, NMPv2, CXE, etc. I mentioned it in another comment. I'm going to upload to Internet Archive soon.
This is officially my favorite thread on HN, ever. Just waiting for rothgar to show up, sorry @rothgar, I should've hit you up for some Halo while I was in Seattle (and the k8s space).
NHE looks interesting, they seem to have modified all clients to make the host player teleport above the map. With the current protocol, it seems like it is impossible to achieve this without modifying the guest clients as well. And by modifying all clients they are also able to add extra maps and make gameplay changes.
