So here is a workflow that will work! :)
In the JSON, set the Textureimportance to "High".
Set the other importances to "Low".
In CC3, send your character's head diffuse to an image editor using the button below the texture thumbs.
Scale it 300-400 percent, and save it so it loads back in CC3.
Export your model with the InstaLOD "All".

Note that you can play with lowering all the texture resolutions in Skingen, then set just the head higher to get it bigger. You can also remove all the textures you do not want represented in the UV (to make additional space). So if your character never opens their mouth, delete all textures from the tongue. Then just delete the teeth from the scene altogether.