There are several hardware components to this project, including two USB peripherals. And the embossing surface poses quite a few challenges.

For the custom chording keypad, I am planning to use either a Teensy board (http://store.hackaday.com/products/teensy-3-2 , http://www.pjrc.com/teensy/index.html), or something like an Arduino Micro (http://store-usa.arduino.cc/products/a000053) that can easily masquerade as a HID keyboard. I will also need some low-profile buttons to use as keys. I will probably get a few different sets of 4 to see how they feel. Most keyboards have about 19mm between the center of one key and the center of the next. These 16mm buttons may be about the right size: https://www.adafruit.com/products/1504. These Tactile buttons are fairly small for a keyboard, but the LED backlighting is rather neat: https://www.sparkfun.com/products/10440.

For the embossing surface, I am considering a Raspberry Pi 3 as the main controller. It doesn't really need that much processing power, but the integrated Wifi and Bluetooth would streamline development of a wireless version. (https://www.adafruit.com/products/3055). As far as the electrically controlled air valves, I found quite a few parts that won't work :-). Most of the options are more than an inch long, are rated for over 100 psi, and cost $20 apiece. I have started to plan out the 25 “4x2 arrays of tiny balloons”, and I have a few ideas that would use a thin balloon skin sandwiched between two pieces of plastic containing 8 holes. With a few 3D printing experiments, I should be able to prototype that portion. I think the big challenge here will be finding 200 small electrically controlled air valves…

Have a great day!

P.S. I am still writing out the Chord Map. It takes a while to lay out all the key combinations in a succinct and useful way.