A brief update. I now have most of the components needed for the tiroir. Arduino development environment is installed and I have sketches that check switches and read analog inputs. Have built and tested hardware debounce for the switches (RC filter leading to a schottky trigger), which works. Have some rail-to-rail op-amps to buffer the analog inputs so they read faster (the capacitor at the input charges faster).
I implemented the red/green signalling using bicolor LEDs. These have three pins, a common cathode and one pin each for red and green. If you set both high then you get amber. I plan to use this, when the Tutti switch is on, setting the oscillators which are used to amber and the ones which are off (petit gambe and souffle) to dark, then restoring the previous values when Tutti is switchd off again.
Here is my wiring diagram for the Arduino Mega2560, it uses almost all of the available pins.
There is space for later expansion (clavier/bague switch, keyboard shake vibrato, and I currently plan to use a second Arduino to measure the bague position with a 10-turn 1k wirewound pot and some pulleys, feeding into an external I²C 16-bit ADC. Or I could use a rotary encoder and count pules - neeed to experiment.
Plan for the weekend is testing the buffer op-amps, and thinking some more about the movement mechanics of the Touche d'Intensité and how to hook that up to a rotary potentiometer.