Do you know of such a cheap e-paper screen with resistive multi-touch digitizer ?
It's a good idea, as it makes it simpler to build the keyboard.
something on top of the e-paper to "materialize" the key and have the feel we need (a "push button" feel). This can be a transparent layer added on top of the epaper which has bumps at the places where the key will be displayed on the screen.
Then, a driver must be found that can handle multi-touch (at least for key combinations) and palm detection, and some level of code on the device to update the e-paper screen with the current layout (the image could change when symbol or shift keys are pressed, if the screen is fast enough which I am not sure), and to decode finger position on the touch screen as key presses.