Are you sure about this? I don't have a tremendous amount of experience with Steam, but every game I've ever had with it has allowed offline LAN play, and even allowed quite a few people to LAN using a single person's account. Granted, most of the games I've played with Steam have been older FPS games like CS, CS:S, Day of Defeat, HL . . . but for all of them I've played on a LAN with no internet access and ~10 people per account.
I was under the impression that offline LAN account sharing was by default allowed with Steam for most of their games, though again I'm only moderately experienced with Steam.