jME3 + JBullet in sync with home-made, JSON based object tree?

Sorry if this was answered before/can be answered through looking through the docs, but is there a listener that is triggered by JBullet modifying an object's position?

Thanks in advance.

Not really, you could create your own PhysicsNodeState and create a subclass of PhysicsNode where you use your state.

The MotionState is only updated by bullet when the position changes.