Versions:
Construct's Armory: 0.0.17-b
Tinkers' Construct: 2.9.1.65
Mantle: 1.3.2.24
Forge: 14.23.3.2655

Observed Behavior:
your second layer of skin (hat, jacket, pants, etc) do not appear in the Armor Forge/Armor Station GUI
Expected Behavior:
your second layer of skin (hat, jacket, pants, etc)would appear in the Armor Forge/Armor Station GUI
Steps to Reproduce:

1. Open your Armor Station or Armor Forge
   
   

Figured it out! Just had to copy some of the model data when the fake preview player is initialized.

Hm, interesting. Not entirely sure how this is all happening, but I'll look into it.