Never tried graphite for a large area, but I have experimented with the same idea on smaller areas and it seemed to work OK. SNJ makes an aluminum powder that's used almost exactly as you've described. You rub it into the dried paint and polish it up to a nice shiny finish. There's a similar product sold in better art stores that works well too. It's a metalic pigment that comes in about a bezillion different shades and colors. It would work very similar to what you're describing.
I think the trick with these types of techniques is always how you seal them. Putting some sort of sealant overtop usually changes the color/texture of the NMF. That's where Alclad2 seems to have an advantage because they're quite resilient.
Murray