I was seriously confused by Hakata's absence when I first skimmed the artbook. After all, Nihongou--the next to make his debut in an event--was featured within its pages. (Much to my surprise and utter delight.)
When I brought up the matter elsewhere shortly thereafter, I was informed that Hakata was something of an "unplanned pregnancy" among toudan.
With this current Underground Treasure Chest  - Easy" event singling out rare Awataguchi school tantou, I couldn't help but be reminded of a plausible theory I developed to explain exactly how Hakata came to be. There's no way Hakata Toushirou's unique function as a "koban multiplying" tantou was some random last-minute addition to his person.
Personally, I believe his special ability was meant for Gotou Toushirou. Though he was introduced later, Gotou's roots as a prized possession of a wealthy banking family make as much if not more sense as Hakata's prominent ties to merchants being cause for this trait. I presume Gotou's unusual length and physical appearance outshone any historical details however; when it came time to conceive this particular sword boy, the creative team decided to take things in an entirely different direction, instead fashioning Gotou Toushirou's gijinka (anthropomorphized character) after the "tough" older brother type (whose only desire is to grow taller) to reflect his blade's slim body and keen edge.
As for Nihongou's inclusion in the Kenran zuroku, the production staff probably had him fully prepared for implementation months prior; possibly as early as January 2015.