I seriously doubt it will have backward compatibly. Everything seems to point to it having a different architecture, and I doubt Nintendo will increase the price of the NX just so that they can put chips in it to enable bc.
They are both ARM chips and the NX is massively more powerful. I guess it could be hard to emulate the fixed function shaders of the 3ds with programmable shaders, but if they can solve that 3ds bc should be doable.