Well, as it turns out, the shipping version of Halo on the Xbox does contain a flamethrower after all, although you need a modded Xbox to use it.

Phforslayer (who wrote Aquaduct, the Mac OS X tunelling program that allows Halo to be played over the Internet) posted onto HBO's forum some screenshots he'd made, included one of the flamethrower and mentioned something called a "gravity gun". Macegac later posted a tutorial on how to get both weapons, and BOLL posted a film of the flamethrower in action. We're hosting a copy of the flamethrower video here at Rampancy.

And, as usual, extra thanks to Louis Wu at HBO, which is where we found out about all this.