The only way I see to implement bots in Halo is if someone writes a "bot client". What I mean by this is you'd have a program on your computer that'd simulate, say, 3 'players'. You tell it when and what game to connect to. When it connects to the LAN Halo game, it acts as if it's three actual players connecting. It'd be a bitch to do, but it's certainly more feasable than figuring out how to inject the AI from single player, then hack 'em to do paths or whatever.
EDIT: A good base of thought for this sort of thing is the bot mod for Quake 2... I think it was called EraserBot. All it did was 'simulate' players to connect to a server.