Floris Great news! Looking forward to seeing how this progresses.
Regarding the d-sub connector and cable, that should work fine for carrying the signals from the touch surfaces to the Trill Craft. The only issue could be that the cable itself will become sensitive and so if it is squeezed hard you might get a little interference in the multiple capacitive channels that are being passed through. Potentially not an issue for you.
In a previous instrument I build with Trill craft I just used pieces of stranded hook up wire (not shielded) which were arranged very messily inside the instrument with no problems with cross talk. Like so:
These bits of hook up wire were soldered to pieces of brass that made up the main touch interface of the instrument. Each piece ended up being a touch sensor with variable reading:
NOTE: soldering to pieces of metal is not an easy task for larger pieces, as you need to get the whole piece of metal hot enough for the solder to melt and bond properly to the metal piece. In the above instrument I used hollow pieces of brass which were relatively easy to heat up but in another instrument I used larger solid brass pieces and making a good soldered connection was really tough. In the end we created a jig to hold the pieces of brass while we heated it up from both ends with two big industrial high-power soldering irons!!!
Might be a good thing to bear in mind for your project. It's not 100% necessary to make the connection through soldering. So long as there's a good conductive bond you'll be fine. Screw terminals, clamps, conductive tape or conductive glue are some other options.
The Euro-trill patch was actually written in c++. The brass pieces on that controller are soldered too although it's hidden behind the panel. Here's a photo of the wiring at the back.
In case you haven't seem this tutorial, here's a lesson as part of the Pure Data course which uses Trill Craft to trigger samples: https://learn.bela.io/tutorials/pure-data/sensors/capacitive-sensing/
Let me know if you have some specific ideas for patches with Trill Craft and we'll see what we can do. Fingers crossed for customs. Things have been going better the last couple of months.