I'm looking for either a 3D model or an image file over which I can apply my own custom graphical elements, such as eyeliner or lipstick.
In the ARCore docs, the solution to this issue is very well described. You can get either an FBX file or a PSD template, over which you place your own elements.
From what I can tell, the principle of ARCore and ARKit are very much the same - there's a standard face mesh which gets contorted to the shape of a detected face, however, I'm unable to find any such materials using Google.