They're just blocked from OTA updates, you can flash whatever you want on the sdcard and then unlock OTA. (I did this yesterday with an aliexpress version x4)

You can. But if you build your own stuff, you can also easily lock yourself out. That whole mechanism that tells the software to look for a new bin file on the SD card and to use that to update is something that needs to be contained in your software. If you forget it, or you get it wrong, the thing will be bricked.