Being as in to the NasuVerse as I am. Some of this fits. Like Nao would work as Sion, But that's where it ends. Shizuru should be Arc. Natsuki as Warcuied just doesn't work. Natsuki works better as Ciel. And none of them work as Rider.
I think that the artist just wanted to put Shizuru in her outfit. If you really wanted to offend make Haruka Berserker. Ability wise Mikoto makes a better Saber. And Mai could work as Sakura but Shiho does fit the role better.