Just for the archive:

After learning a bit more about forth, the words and see commands, I found
out that I needed to write 0x18 to index 0x96 of the via-chip with the A8/A9
combination. This is somehow descibed in the app-notes, but I couldn't
believe it. So now I have access to the config space using CF8/CFC.

Thanks for all hints
  Alexander