Mm...yes Face ID will still work even if it is laid flat on a desk so long as your face is within reasonable range of the Face ID sensor array. Since the new Face ID works at any angle, lay the iPad Pro flat to your side (around 45 degrees to your left or right and about half an arm’s length away) with the Face ID sensor positioned closest to your face. Face ID is on the power button side of the iPadPro. That way, you can keep the desk real estate directly in front of you free to do your stuff. For faster unlock, set to off “Require Attention for Face ID.” It is an option but not recommended, for security reasons.