The vast majority of a beams stiffness comes from it's outside so adding material inside wouldn't make much difference unless you were going to tension the struts which I would think would be bad for stability. It might help resonance but gluing a lump of wood inside would probably make more difference for the weight. Filling with an epoxy granite mix would be better if the steppers can take the extra weight. Or maybe epoxy granite poured around a wooden core. Or maybe just thicker steel :-)