Could you try ctrl+ ß key, or perhaps some variant of that?
I don’t have a German keyboard (nor am I on linux), but if I change my keyboard layout to German on my macbook, ⌘+- seems to work (and it gives the same keyCodes as / of “191”). “-” is the key two to the left of “delete / backspace” on my US keyboard, thus ß is my guess after staring at this picture from wikipedia of the German keyboard layout.
