Manual focus?
Focus on another part of the face such as the cheek or chin.
I wasn't being glib. With a 2.8 the depth of field isn't quite that narrow, as it might be with an 85mm f1.8, and especially the 85mm f1.4. If another part of the face isn't available on the same plane then most lenses will allow you to manually override the autofocus once you get it, so take the autofocus and give the focus ring a slight turn towards infinity (notching it a little further away). It will take practice, but if you've got nothing else to focus on then that's what you got.
Another suggestion might be that you extend the dof using a smaller aperture so that the glasses and the eyes are in focus? You may get the desired effect at f4 or even f5.6 - or at least not lose much. Getting the eye in focus is likely more critical than the razor thin dof effect.