There is a published method for sharpening microtomes (which make sections for microscope slides, usually between 0.05 and 100 µm in thickness) that sets the blade up on a wooden stand, and uses wooden paint stirrers charged with diamond dust to finish the edges. These are blades that you can almost drop a tissue on lengthwise and the tissue will be sliced open as it falls. And the sharpening uses essentially waste chunks of wood.
There is nothing to say you could not use a cast iron lap to get some sort of shiny surface approaching "machine tool" dead flat. But Leigh's right: It is overkill. Heck, a piece of wood can expand and contract more than 0.001" in a single work session. And so +/-0.00001" is needed, why? Forrest is right, too: The time it would take would not be worth it in the sense that your neighbor (who used emery cloth stuck to plate glass with some water and was done sharpening in 2 minutes) would have an entire shelving unit fabricated and installed while you would still be sharpening your plane!
For a nice flat surface, use water to fix a piece of fine sandpaper (I've used 400, 800, and I think 1500, the black kind) to plate glass with water. You will be good to go, with a nicely flat, shiny surface and a straight edge.
J