I had thought ADB was the only way to unlock.
I have found in the wild US At&t phones with a bootloader only unlockable with an app and code form At&t after verifying the phone has been paid off 100% by the subscriber while they are still under contract.

Re: Ubuntu Touch above, I tried it out early with my Nexus 7 flo but found it to be pretty limited, I place privacy, real GNU Linux, and function on a balance and try to get the best of all 3. Pretty much until there are used Purism or something similar phones available I am in a tough spot for hardware as I think most of us are.