Oh shit, I'm sorry for leaving you hanging for so long. I checked out the works of the people you mentioned and they're doing some really advanced stuff that I probably won't be capable of in the near future.
I think the best method you can go with for now is just loading the custom game animations and then using the kinematics function to modify them.
This video was where I learnt the basics of Fk and ik functions and I'm sure you'll be able to figure out the rest by playing around with the studio or searching his channel for videos related to stuff you need. If you ever need help with anything koi related, you can join their discord server here; https://discord.gg/koikatsu
Or message me on Twitter. Once again I'm sorry for leaving you hanging.