I *do* see the device in fastboot. You just have to make sure you have TWO udev-rules in place, since the device shows up with a different vendorid (the one of goolgle, not ZTE) when booted in fastboot.
Unfortunately, I have bricked my device that's why I can't post you the exact ids. But I think it was 1d13 for Google.
Please note that somehow flashing over fastboot does not work with the ZTE open, erase and flash work but it seems after transfering the data to the phone it somehow aborts flashing.