Maybe there would be enough internal locking for the pieces to remain attached. Also, if you have the piece thin enough in cross-section, you might consider plastering burlap on the inside as you reconstruct it, for bracing. I know this will add to weight, and of course, it also will have to be done with the original pieces damp.
This is my concern too. Thinking ahead, is the mould to be made with hot melt rubber or something which cures at room temperature, eg silicone or latex?
If hot melt (ie reusable) rubber is to be used, you will have to seal the surface with some kind of varnish to lock in the air bubbles, or soak the original in water - which would dissolve water soluble glues used in reconstructing it.
Presumably the piece was hollowed to save weight? If you want the reconstructed piece to have any strength, IMO you need some scrim or other fabric dipped in plaster on the inside for reinforcement, or even pieces of wood or wire mesh.