I did some further reading, and it seems like the method was first described by Da Vinci!
Geometric Fret Layout--the ...View Instructable »
Hi Honio. You can indeed just tie the diode and resistor outputs together. When you select, for example, the resistor output on the switch, the diodes (which are before the switch) are no longer in the circuit. Then, wire it something like this: I'm not sure what would be a good value for the potentiometer? The output voltage is fairly hot (a little over a volt) so probably a 500K pot would be a good starting place? Let me know if it works. Did you build the circuit yet? It's a lot of fun to mess with. http://guitarwiring.blogspot.com/2011/02/simple-guitar-wiring-for-volume-pedal.html
Simple noise maker!