I don't know much about kotodama but I have done a certain amount of chanting practice. One thing that I am very interested to find out is whether there is an aspect of kotodama practice that addresses the issue of how intention influences vocalization.
We all have felt a sensation at times where a word wouldn't quite come out, where it was almost a stutter but not quite, and the reason had something to do with your intention fluttering a bit. Not quite losing your train of thought but more like it was off the tracks a bit.
I find the same issue in my practice when I am not quite fully committed to my posture, position, or movement. It is a very similar feeling. Something about sincerety, and wholeness.
I think one place where the issue with respect to vocalization converges with the same issue with respect to movement on the mat has to do with breath.
I'd be very curious whether this is something that kotodama practice could help clarify.