An example of a way to simplify managing pose layers.
The return value of tachieEyesFile is only ever passed into ImageManager.loadTachie and not used anywhere else. I believe this should be the same for all the tachieXxxFile methods.
If you hook ImageManager.loadTachie then you can return additional data from the tachieXxxFile methods.
In this example, I allow the tachieFile method to return an array that contains the hue along with the filename, and then loadTachie notices that and passes the hue parameter through to the normal loadBitmap files
You can simplify the hair layer the same way if you also hook Game_Actor.prototype.getCustomTachieLayerLoadout and manipulate the layers so that the poses include the hair layer in the right place.
As a further extension, you can use the Game_Actor.prototype.getCustomTachieLayerLoadout hook to add additional custom layers (e.g. for womb tattoo) rather than hooking the draw calls, but that requires a patch to Saba_Tachie.js if you have more than 1 custom layer.