Desiring Q-like powers, I searched for a way to change the gravitational constant of the universe. Unfortunately, my search produced only bitter disappointment.
I don’t know Lua or Defold particularly well, but I do know that Lua has a C API, which got me wondering wether there is any way to access the Box2D World so that I can set the gravity “manually”? How is the Defold extension project coming along? Is there at least a Defold C API?
No, there is no access to the internal structures or the Box2D world at this moment.
The extensions are coming along fine. You can find the Defold SDK reference documentation here: http://www.defold.com/ref/ It’s listed under DEFOLD SDK (ALPHA).
You can set the world gravity in the project settings. 
Thanks, but I’d actually like to change gravity dynamically at runtime. Unfortunately, the project setting is only modifiable at build time.