Under optional files you will find the HeadRear Custom. The esp is marked as light plugin. It contains 5 head parts for the rear, which you can use inside your JSON presets. Open the preset and replace vanilla headrear:

"Fallout4.esm|04D0E9"

with one of the following head parts:

"HeadRearCustom.esp|F9E"<-- Texture 1
"HeadRearCustom.esp|F9F"<-- Texture 2
"HeadRearCustom.esp|FA0"<-- Texture 3
"HeadRearCustom.esp|FA1"<-- Texture 4
"HeadRearCustom.esp|FA2"<-- Texture 5

These headrear parts use vanilla based textures, which are placed in folders:

\Data\Textures\Headrear Custom\Texture1
\Data\Textures\Headrear Custom\Texture2
\Data\Textures\Headrear Custom\Texture3
\Data\Textures\Headrear Custom\Texture4
\Data\Textures\Headrear Custom\Texture5

In case you want to replace them, the files have the same name as the body textures:

FemaleBody_d.DDS
FemaleBody_n.DDS
FemaleBody_s.DDS

so you have to rename your files accordingly.

Version 2 is basically the same mod, with a small difference in the plugin - switching the texture for the rear is done through Material Swap instead of Texture Set. Try this when you experience the following bug - rear of the head reverts back to vanilla texture after loading a save. Otherwise stick to Version 1. I tested both versions, the custom rear head stayed unchanged upon loading. I also learned that Rusty Face Fix will cause the issue mentioned, and this will make you think it's a problem with my mod. Thanks to user Beyond00Cloud for pointing out the culprit. Again, I recommend Version 1.