Your point about the visual keyboard is important. As long as they have a repeatable 1 bit worth of output, you can extract any number of bytes. You can have a keyboard where they can see it, move your finger from one letter to the next, and then when they do their 1 bit of output (e.g. blink) that locks in the letter. For the film lovers out there, there's a great dramatization of this in the 2007 French movie The Diving Bell and the Butterfly. [1]
--