I'm not sure that's exactly human friendly. Can people answer questions like that without training? Would the amount of thought needed create a noise signal?

If they have vision, you could ask "Which of these two sets is the letter in", where the sets were dynamically generated by predictive software.

yeah, I guess you'd start with the first 13/last 13 and break it down in halves from there. It would take only 6 choices to pick a letter. I still think memorizing the binary version of the letters would be faster, and wouldn't take all that long considering the amount of use.

The code is (or can be) the same either way. I imagine after a few days the user would memorize the codes for the letters and skip the prompts.

On a related note, one character per minute is about the same speed as the first trans-atlantic telegraph cable in 1858.

You want a variable length code that takes into account letter frequencies.