it seems like the touch system should be able detect where your main 4 fingers are, and then show a rough virtual keyboard in its space with your 8 fingers on the home row automatically. The only thing lacking would be tactile feedback, but that's solvable. If people are accustomed to seeing the 10 dots representing the fingers, it should be easy to provide small visual cues to indicate if you're about to type a letter that isn't what you want.

I type 80+ words a minute regularly. It would be insanely difficult to type at that speed without the little nubs on f and j keys, as well as the tactile feedback as fingers slide over keys preparing for what they're going to be typing next.