Can I change the directory used for 'HumanoidDescription's?

You can not have a HumanoidDescription do that but what you can do is make a script in ServerScriptService and use this script:

-- Variables
local PLACE = game.ReplicatedStorage
local NAME = "NameOfTheItemHere" -- Name for the accessory/shirt
local ITEM = PLACE:WaitForChild(NAME) -- Gets the item

-- If you are doing a face then leave everything up there
local TEXTURE = TextureIdHere -- You need to make an image then upload as a decal and get the ID and replace TextureIdHere with the ID
-- End variables

-- Events
game.Players.PlayerAdded:Connect(function(plr) -- When a player joins the server a function is fired
if ITEM then -- If there is an item then its not a face so let's parent it to the player's character
ITEM.Parent = plr.Character -- The item is parented to the player's character
else -- There isn't an item then its a face
plr.Character.Head.face.Texture = "rbxassetid://".. TEXTURE -- Sets the texture of the face to the given ID
end
end)
-- End events