Ok, fixed, CoreText is now using 1024 bytes less than FreeType.
Not the win I was hoping for. Clearly I wired this up to the wrong place, because what I really want is for Godot to stop loading a full copy of that fat Emoji font - and it is doing it with gusto.
Look at this Absolute Unit of a font:
Apple Color Emoji
Created a data provider with size: 188,589,668 bytes