There are pretty much two ways to do this, neither one works well:
1) Overlay multiple transparent keys on a single display. This works OK in concept, but is hard to do mechanically gracefully. You would also end up with parallax issues with the touch functionality, if you could even get multi-point touch functionality working on key-faces like that.
2) Keys with small displays and captouch sensors in them: The display tech isn't quite there yet for this, AFAICT. I've seen demos of small, single-color OLEDS embedded in buttons, but none with touch overlays and nothing with a higher end display in them. Additionally, due to the bezel and gaps between the keys, it would be very difficult to get a consistent image across multiple keys/displays.