I just had the opportunity to try out the "Live Preview" untethered on a Phase One IQ180V system. I spent a few minutes with it (will have more time later), but the performance is pretty good.
It is accessed via the lower bottom right hand menu (see image below). You click on the movie camera icon, then open the shutter and the image appears.
We shot out our window of our office into the daylight and we had the aperture set @ F11 and the preview was not blown out at all. We then put a ND filter and set it to F2.8 and it worked well.
You can zoom all the way up to 400% in the Live Preview mode just like you can do in the full screen preview.
More info to follow as we get some more time to explore it.
But in my opinion it is usable and exceeded my expectations.