Yeah, I tried to use that face as well but I'm running into a couple of problems. My .ini files are a bit weird, and maybe it wouldn't happen with your game? You would have to replace Kokoro's face in chara_common using Archivarius, extracting Miyako's head first then using the swap option. Then, create a DLC using a "bald" hairstyle (since Miyako has hair included in her model).
Unfortunately any change I make to chara_common.bin does not seem to be detected, I don't know if that's because of the tool or me.