Okay, I can officially say that the genuine mad geniuses at https://insignia.live/ have brought original XBox Live back from the dead. I am in Halo 2 matchmaking For Real. On an emulator, no less.
It took some fuckery to get the files just so, and I know not everyone has the patience for that, so if you want in on this but can't be arsed, email me and I'll schlep you some things.