Flexible keyboards like this, this and this are super cheap and readily available. I wonder if we could use them to make an alternative keyboard for people with physical, visual, or cognitive disabilities?
I'm thinking we could turn the keyboard over (keys down) and glue it to a thin board. I'm guessing the back of these things is pretty smooth. You'd put your tactile or visual overlay over the back of the keyboard. Now when the user presses, multiple keys will likely be activated. But we can handle that with a special keyboard layout as described here for Windows. I'm pretty sure there are comparable capabilities for Macs and Linux.
We could even define some sort of simple overlay maker that makes it simple to define and use specialized layouts. I'm guessing we could only support relatively large regions with this simple cheap approach.