I'm interested in trying to modify a set of armlets a la Strarf or the Valkyrie girls for a Shinki that doesn't come with them by stock. Has anyone had success in doing this? I'm a super noobish kitbasher, and don't really know how I'd go about creating a backpack structure to mount something like GunPla or other model arms.

Well, all shinki come with a back-mount plate thing that provides a good mounting point for things like that, so I'd suggest just trying to mount the base of the unit to that.

Xehn's got the right idea; all Shinki come with a back mount of some kind. Those that don't use the default one will still come with it, and it makes a decent base for customization. Depending on your Shinki and how much of their armor you want to keep, you might want to use the specialized one if they come with it; Eukrante, for example, has a better-than-normal back assembly for mounting arms. With the default, though, I'd recommend using the "crank" pieces off the extra sprue to get the width normally found in extra arms.