Phase One has intentional blocked support of the 645z. However, Alex Munox has written a nifty little javascript called "Capturefix" that (for now) allows C1 to import 645z Raw files. The colours are all f'ed up, so you're on your own there, but yes, I can confirm it will import.
The link to the script download is here:
Capture One Fix adds support for 645z images - Alex MunozAlex Munoz Lifestyle/Fashion Photograpy
Note, you need the SDK version of Java to run it.
If viewing of files during tethered shooting is your only concern, all of these programs will support jpegs from the 645z. This also speeds the preview process a lot. Send a Large, one-star Jpeg to the computer and save the RAW to card if speed is your paramount concern.
- N.