I tried it on my z lite which was bootlooping. In the middle of the firmware upgrade i saw a red 'X' for a split second then the animation stopped. I left it for a couple of minutes then it went back to the firmware upgrade page. It was supposed to restart the watch once the upgrade completes. But since it went back to the upgrade page I'm guessing it didnt go well.
There is a restore tool for the z1, an upgrade for the z1 and another for the z lite. Maybe you can look in to those and help you compile an upgrade.zip which will work for the z1 and z lite. And hopefully help me bring new life to my paper weight. I would love to help with a logcat but i don't know how to get 1.