I am going through this right now... I really wanted an 85mm 1.4G but according to reviews it will put focus completely behind the subject from f/1.8-f/5.6 when closer than head+torso distance. In headshots if you target the eyes it will focus somewhere on the temples. I have no idea if it really matters in real life. So I am stuck with the 1.8 G.
Edit: A focus screen with some kind of prism mechanism would let you manually focus, at least, by holding down DOF preview. Richard Haw describes cutting down and shimming off-body focus screens so it's possible. Actually now that I look it up, whoa, an off-the-shelf one exists on aliexpress ("Dual 45 degree Split Image Focus Focusing Screen for Nikon D1 D1H D1X F100 PR129")