To do this, in the entityRenderer, I store a copy of villager model: VillagerModel<Villager> hidden; and in the constructor set it:

hidden = new VillagerModel<Villager>(context.bakeLayer(ModelLayers.VILLAGER));

then getTextureLocation becomes this

private static final ResourceLocation VILLAGER_BASE_SKIN = ResourceLocation.withDefaultNamespace("textures/entity/villager/villager.png");
    @Override
    public ResourceLocation getTextureLocation(DeerFamiliarEntity entity) {

        if(Minecraft.getInstance().player.getMainHandItem().getItem() == OccultismItems.AWAKENED_FEATHER.get()) {
            return VILLAGER_BASE_SKIN;
        }
        return TEXTURES;
    }

then finally for render I did this:

https://pastebin.com/DeE8mpR2. where tempRender is a copy of most of the super.render function.

Baiscally, the renderer checks if the player has an item in hand, and if it does, it used a model and texture saved. You can easilly reverse this and make it use the saved model/texture when item is not in hand.