I'm sure a lot of those are connected to the character models as said above, but I can't imagine it would be too difficult to open things up a little bit for characters with similar models. At least on default sit/stand poses. Miqo and Au'ra or Midlander, Elezen or Highlander and Viera, and Highlander and Roe for example would probably be relatively compatible. Sorry Lala folks... I have no idea what model would work with you lol. You're pretty unique. That might be a bit too fiddly though.