Has anyone researched or built an aircraft that controls pitch and roll by pumping liquid within the airframe?
The aerodynamic forces resulting from surface deflections are orders of magnitude larger and faster (in their rate of change) than the gravitational forces obtainable by shifting the centre of mass. 

Additionally, as OP seems to realize, yaw control would not be feasible with this method.

So overall it would not be impossible, but it has no clear advantages to outweight the heavy disadvantages. I am also unaware of any prototypes or tech demonstrators using this method.