I’m still pretty divided on this idea, between two modes of implementation I’m calling The Steed Way and The Universe Way. On the first hand (The Steed Way), I’m envisioning using this third Code to represent the character’s relationship with their Steed, with the scale going from one extreme (“My steed is just a tool and nothing more.”) to the other (“I am the steed.”). Given the intended portability of the game, this could of course be a Bike in the default game setup, or a Horse, Spaceship, or whatever in alternate home-made scenarios.
Doing it this way would keep the three codes thematically similar, each one representing the character’s relationship with something external: The Team, the Sword, the Bike. These three relationships would be important to the situation in the game, and each would have social rules (the Codes) and expectations for maintaining good standing. The rules of the Pack dictate proper behaviour for members, like a motorcycle club charter. The rules of the Sword dictate proper use of the weapon, to the point of becoming the weapon. The rules of the Steed cover care and maintenance of your ride, as well as rules for proper traveling and such etiquette.
On the other hand, following The Universe Way, the Highway could also be a metaphor for the character’s relationship with the Universe at Large. At the low extreme, he again is just a tool-user, with no relationship to the greater world. At the high extreme, he has become the zen master, one step away from true consuming enlightenment, and everything else is just so much unnecessary time-wasting.
These two different ideas, I’m not sure if there’s a way to implement them both without overwhelming the player with too much Thematic conflict. Both implementations have a similar spiritual component, although the Universe Way would have heavier implications in this regard.
Each of the three Codes is intended to clash both within itself and with the other two. The innate conflict of the Pack is about keeping good standing within the ranks, while not getting consumed by the group mentality. Within the Code of the Sword, the character must find a good balance between using the sword to kill, and becoming a creature of pure emotionless murder in the possession of a Master. Likewise, the internal clash within the Code of the Highway could involve the balance between using the steed and becoming one with the steed, shedding your own identity in the process.
The inter-code conflicts, then, are a matter of balancing the extremes on their scales. One who has become consumed by the Pack Mentality will suffer on the other two scales, as loyalty to the team will be more important than skill with the blade or mastery of the steed. One who becomes the perfect weapon will be cold to his companions, and careless with his steed. One who becomes a melding of man and steed will be isolated from friends, and possessed of less reason to kill.