Enable "OEM Unlock" via build.prop edit!
OEM unlock is not a line in the build.prop file or any other user accessible file that we know of, it is a hidden flag somewhere (probably encrypted) and not in the ROM itself (flashing a custom ROM or completely wiping all visible partitions does not change the setting's state). Unfortunately, if you can't somehow access the Developer menu, you will not be able to unlock the bootloader.

The answer is that as of now, this is not possible with this device.