Hi,
This is an interesting topic. I just wanted to add a few ideas that may or may not be useful:
I am currently using a homemade midi breath controller – it works very well, and could be integrated into a portable instrument, existing or homemade. I did consider doing this but have had so much fun using it with a separate keyboard - comping with my left hand on another keyboard - that I have stuck with this setup.
Whether you want a self-contained wind instrument or a more traditional breath controller+keyboard setup, I would recommend going the midi controller way and not build a small synth into the instrument. There are so many fantastic soft instruments these days that can handle midi breath controller input, either via a dedicated breath controller mode or via simple cc2 messages linked to specific parameters, typically the filter cutoff of a soft synth. I have had luck using for instance the ROLI synths on my Mac and the SWAM instruments, Moog synths, an accordion app and the Magellan and Kauldron synths on my iPad.
My design is relatively simple, and the components are small:
Mouthpiece/tubing -> 5 V analog pressure sensor circuit -> Doepfer Pockets Electronics (converting analog signal to midi) -> External synth (software or hardware)
This way I have avoided programming (the Doepfer fixes that). I built the sensor circuit myself, but I believe that you can buy breakout boards with the whole thing on them. The most important part is being able to calibrate and adding gain to the pressure sensor which is not very accurate by itself – or was not in my case. (Note the sensitivity/offset "pots" that they mention in the KeyWI description). Sensor accuracy seems to be a very common problem among engineers who supply many, rather simple circuit diagrams on the internet which solve this (search "calibrate pressure sensor").
Or you can simply buy and modify, for instance, the TEControl BBC2 breath controller which seems to be a very capable midi breath controller. This will be more expensive, however.
I can only recommend going the pressure sensor/breath controller way, DIY or not. It is very rewarding - and very, very addictive.
All best
MortenL