@Egon: Yes, you totally understand what I want. This thread is just about getting a working prototype. Adding functionality and changing configurations is of later concern (or at least not my concern!). And please, no more discussions about batteries!

*ON-TOPIC starting point*
Okay, the first concept for pcb wiring of the keyboard is this:



Don't know what all of these lines mean? Check out the links in the first post of this thread.

This will provide a working solution for a proof of concept. There maybe more efficient ways to organize the wiring and I challenge you all to design a better one! Next step is to use these drawings and the gpio chip manual to design a full pcb. This will be connected to my RPi, so it is all about getting it to work and not about smoothing the edges.