Is there a way to manually control vertex/face normals to get a more consistent toon shader? I watched this video about the art style of popular fighting game Guilty Gear Xrd, and something which really interested me was the method that the artists used to get rid of ugly automatic-normal artifacts. They would manually mess with the normals so as to make parts of the model that are round shaded as if they were flat.

The example shown in the video is of a character whose face's normals are grouped on each side, on the chin, and under the eyes so as to make the toon shader look more like a traditional 2D anime.

With modified normals:



Without modified normals:



Notice that this sort of ugly artifacting happens when a toon shader is applied to Suzanne; look at her right eye and nose.



This would be very easily fixable if there were a way to group the normals on the left and right sides of the face.
Based on the other answer I created a Blender add-on for this exact purpose called TiNA which allows you to more easily transfer normals between objects. TiNA basically turns the somewhat overwhelming Data Transfer Modifier into a set of more comprehensible operations.
There's a special branch for those using Blender 2.80 beta.



 Hotkey           | Operation 
------------------|-------------------------------------------------------------------
 Alt+N            | Transfer Normals from selection to active object
 Shift+Alt+N      | Transfer Normals from active object to all other selected objects
 Ctrl+Shift+Alt+N | Clear Custom Normals data for entire selection
 Alt+W            | Wrap Normals


In your case you could transfer the normals from a normals source mesh (based on the actual face mesh) by selecting it, then selecting the target object and hitting Alt+N.