Tests with adding a custom facial expression as the existing ones for LEGO are too limited. I added the facial features using a narrow non-permanent OHP (overhead projector) pen which I can rub off with a tissue to change the expression. This means that the character can have the full range of expressions required for production (limited only by the style of LEGO minifigure faces).
In the photos below I show the comparison between the custom faces and default printed face. NB: The latter is just a basic photo to show this and both will be differently lit etc. in the final.