On the DF, you have to raise the mirror and open the shutter ... camera and back do not communicate about Liveview.
Easiest is to set the camera on a long exposure time, 30 seconds or so, and trip the shutter. Save that as a custom function, then just switch back after focusing.
Seems to work pretty good, but not sure LV focusing on the DF will be more accurate than just letting the camera AF.