with a section of plate glass you can do the old machinist trick of printers ink and paper. You just have to figure a way of lowering the glass completely evenly (spirit levels, even of the torpedo version, can help here) anything that touches first needs to be ground down slightly and go from there. There is also taking the pieces to a stonemason and having them level and polish. My question is how much dampener are you going to put between any Mill/Router that you plan on mounting on it and the tool. Reason being that stress will build up in the stone and it WILL split along any inherit fault lines within the stone (ask me sometime about a number 1 diamond wheel and Obsidian). Great idea if you can control the vibration reaching the stone or give it an ability to transfer it.