Some wire placing and code writing later I was able to access the SIDs registers and now I wanted to make some music…
The idea was: use a PC based C64 emulator and replace the SID emulation with the SwinSID connected via USB. First I tried jsidplay2, but I had no luck understanding the timing for the emulated 6510. Luckily I found kernal64, a C64 emulator implemented in Scala.
I had never before used Scala, but as it is Java based I was shortly after able to do this:
Please find my patched kernal64, the Arduino-sketch, schematics and some instructions here.