Hi Mark,
by FACS do you mean Facial Action Coding System? (I found more meanings for this acronym)
If so, the best way is to have a camera rig around the head with known cameras positions and fixed head. You can take a video of facial movement in high video quality or take images in same time. This could be very similar to a full body scanning, but you will scan just a head.
You will need to have some cameras around the head on a fixed rig. And also placed some control point with known coordinates or distances between these points. Then you can use XMP’s workflow to process your images into models.
For some inspiration you can watch these tutorials: https://www.youtube.com/watch?v=5VyYLaNxHz0&t=61s, https://www.youtube.com/watch?v=E2QCtisvdDc&t=21s, https://www.youtube.com/watch?v=dKX48fLuyv0
For each expression you need to create an own model which can be then merged into movement in 3rd party software.