ADDED You can now set a race's GUI item to a custom head! Simply set the material to "skull:" followed by a value from a custom head. I'd recommend using
Minecraft Heads. Pick a custom head and scroll to the bottom, copy the bottom value (under Minecraft-URL) and paste it after "skull:", like so:
Code (Text):
skull:e2274bbf9f179002a1127c1730e470add89c26655b0556b484778f3430086424
