To the manga, I found only this, not sure if it helps: "The next Young Gangan issue, on sale on April 18, will feature the Jin: Anime Seirei no Moribito Gaiden manga written by Miki Nakae. This side story will center on the character Jin from the Moribito - Guardian of the Spirit (Seirei no Moribito) anime and will feature the art of the anime's character designer, Gatou Asou." Since it's a side story, I don't know if this is the same, as the one mentioned by the author. Still, by title it's the same, if you look at the site of Gangan.
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"The next Young Gangan issue, on sale on April 18, will feature the Jin: Anime Seirei no Moribito Gaiden manga written by Miki Nakae. This side story will center on the character Jin from the Moribito - Guardian of the Spirit (Seirei no Moribito) anime and will feature the art of the anime's character designer, Gatou Asou."
Since it's a side story, I don't know if this is the same, as the one mentioned by the author. Still, by title it's the same, if you look at the site of Gangan.