How to load .ht files?

I am trying to save/load a .ht file to use as a “master” template for several hundred animation files that are auto-mapped via mecanim. (The mapping generally works great, but there are some issues with the finger joints on some clips and, as you can imagine, manually mapping the finger joints on several hundred files is laborious and prone to error).

The unity docs claim that you can save/load a .ht file but I am not having any luck getting it to work. When loading the mapping file nothing happens or only the root joint is mapped.
Any help is much appreciated.

Saved human templates have hard time loading yes. But you can instead do the exactly same thing with: Humanoid → Copy From Other Avatar