Yes, in some fanfics Naruto might know the Hiten Mitsurugi. Fanfic writers often like to cross different elements from various anime or manga. They could create a scenario where Naruto, being a curious and adaptable character, somehow comes across the Hiten Mitsurugi technique and learns about it. This could be part of a story where Naruto travels to different worlds or meets characters from other series.
He could learn it from a crossover character. If there's a character from Rurouni Kenshin in the fanfic, that character might teach Naruto the Hiten Mitsurugi. For instance, if Kenshin himself is in the story, he could see potential in Naruto and decide to pass on his knowledge of this sword technique.
One popular Hiten Mitsurugi - Ryu Naruto fanfic could be one where a character uses the techniques from Hiten Mitsurugi - Ryu in the Naruto universe in unique ways. For example, they might combine it with ninjutsu to create new combat styles.
Well, in a fanfiction world, it could happen if there was some sort of cross - dimensional or time - traveling event. Maybe Naruto stumbles upon a portal that leads him to the world where the Hiten Mitsurugi - Ryu exists. There, he meets a master of the style who sees potential in Naruto and decides to teach him.
It can change the power dynamics. For instance, if a minor character masters the Hiten Mitsurugi - Ryu, they might suddenly become a more formidable opponent, changing the course of battles.