Update! Version 2.
Now it's not required to flash/downgrade droidboot.img. But you still need to flash one of compatible ROM.
Currently compatible versions are:
For ZE551ML: v184.108.40.206, v220.127.116.11, v18.104.22.168 and future newer versions unless opposite mentioned.
For ZE550ML: v22.214.171.124, v126.96.36.199, v188.8.131.52 and future newer versions unless opposite mentioned.
For example, you can flash full ROM v184.108.40.206 and then use unlocker v2.
Just unzip everything from attached archive and then start unlock_one_click_v2.bat
If you've enable USB debugging then phone will automatically reboot to bootloader and will unlock the bootloader. If you didn't enable USB debugging, then you need to reboot to bootloader manually (Turn the phone off and disconnect USB, then press Power+VolumeUp keys at the same time and keep it pressed till you see ASUS logo, then you may connect USB cable).
Either way, unlock_one_click_v2.bat will proceed to unlock in fully automated way.
If initial boot splash screen become inverted, then bootloader has been unlocked!
If you don't like white boot screen, then you can flash original black one:
Originally, it's developed for WW firmware but should work on any region.
Works on CN firmware. Confirmed by @corrsea
Based on unlock method discovered by @shakalaca
Based on fastboot hack by @social-design-concepts