It is, of course, possible; any iPhone can be forced to enter the restore mode or the DFU mode (don't remember which one) and be erased/reflashed via iTunes. I

It is, of course, possible; any iPhone can be forced to enter the restore mode or the DFU mode (don't remember which one) and be erased/reflashed via iTunes. It won't get unlocked by that, because as soon as the new iOS takes over, it will detect the activation lock and require the Apple ID password to be provided. Also, going through iTunes erases all the data. But presumably a custom DFU update can only replace the OS without replacing the data. The iPhone doesn't try to protect against valid updates, and will happily run anything signed by Apple; the locked/unlocked state is only about the encrypted user data on the device.