When named with a name tag, the mob will try to search online for the matching player skin online.

The offline alternative will be to have them to use custom player skins that can be added.

I don't know about the online thing, but custom skins is something I wanna add.

I would imagine the naming thing will work similar to vanilla head, grabbing the corresponding uuid when named.

In a sense the mod have custom skin, the mode is basically reading player skin file in mod folder assets\humancompanions\textures. You just need to unzip the files, replace each skin (same name) and reload game (I checked the code, mod is reading each png by name). Maybe in the future 1: move the texture folder to mod config folder, 2: scan and iterate texture folder instead of getting result by name...