Joel, g is still the free-fall acceleration of bodies near the surface of the Earth, which istself is due to the pull of the whole Earth. I only assume Newton's Laws of Motion and a state of Equilibrium to derive N = mg. My students understand it. Try avoiding the use of "weight" when students constantly hear or read about "losing weight." Individuals leaning against the wall require friction and horizontal forces for equilibrium; I do not consider such, only vertical forces.