I don't have much to show you - sorry - but I spent a couple hours during the night working with the Z shooting the Milky Way both with and without a tracker (always with in-camera long-exposure noise reduction turned on - the temp was about 60 degrees F). I was also shooting a Nikon d800e (more than 50,000 images of stars shot with it this past year), so my thoughts are based on comparing to those files.
Just a couple of quick thoughts after looking at some of the RAW files this morning. ISO 51,200 is usable for Milky Way photos with the Z (after noise reduction in camera and in ACR), and seems to be about the same as ISO 6400 on the Nikon - so three stops better - can that be real? I was not prepared for how good the files were - I was a big skeptic. I was shooting with the 35mm A lens at f4, 10 seconds, ISO 51,200 without the tracker (bad coma on this lens around the edges, but usable at f4). Files shot of the Milky Way with the tracker at lower ISO (12,800) are pretty darn amazing, but I can't always use a tracker so it is important to me to be able to shoot without one. With only slow lenses available for this camera, really high ISO is mandatory for "real" pinpoint star photos (the 500 rule doesn't really apply for pinpoint star photos and high-res sensors). Nothing will be able to match the 14-24mm Nikon f2.8 at 14mm, but still very impressive results for a medium-format camera!
I will also add that I literally cut open the box at midnight while standing in the middle of a dark field, stuck the camera on a tripod and started taking pictures. I've been using a 645D for a couple of years for general landscape, but of course never for stars. The Z was kind of clumsy to operate compared to the D, but I'm sure a lot of that has to do with knowing where the new buttons are and how they operate. Results will be even better with the 25mm lens I'm sure, I just wish it was f2.8. I'm a happy camper though.
Here is one of the untracked 51,200 images, 10 seconds, f4, 35mm, with just a little processing in ACR.