(I moved it down here to blood & guts.)
The answer is, yes, totally built-in hackable, from the start.
I started the design by making, basically by gut, the brainer character playbook. That gave me a first-draft set of stats and basic moves. From there, the process of making a new character playbook IS hacking the game. I made the angel second, and it's really correct, historically and design-wise, to see the angel as a brainer hack.
Once I had the basic moves and a couple of playbooks for a core, I created the rest of the game solely by hacking them. I always figured that anybody else would be able to look at the game and see how I'd done it, see how they could do it themselves if they wanted.
After we'd played the first session of the first playtest, Shreyas and Elizabeth (who were in that playtest group) started hacking it. They brought me a beautiful new non-apocalyptic character playbook, it was tarot-based, and it had some ideas in it that I dearly hope see print someday. There were certain circumstances under which you had to change your character type, it wasn't strictly voluntary like it is in Apocalypse World's expanded improvement. Anyway, that was the first confirmation of the game's hackability, and it came pretty much alongside the first confirmation of the game's playability.
-Vincent