My next step was to build a working core (lpc17) with some DINs and DOUTs. After some troubles ... I succeded.
I am really astonished, this device is the least troublesome device I have encountered so far. No extra drivers, just plug and play, and the controller can be unlugged and reconnected while traktor is running, no crash, not even a lag!
By the way, the midibox-site is really awesome and must be really a lot of work, so thanks to everyone and especially Thorsten!
A pic of my core with a DIN and a DOUT, even got a display for it...