You are correct that the guard pin doesn't do much and can easily be omitted. This was a klugy way to implement a technique which is used with op amps and other high impedance (low current) circuits.
You can now use the [[Arduino Playground - HomePage capSense library]] and use any pins you want. The results will vary a bit from pin to pin though.
But if you do want to make the wire insensitive and only the foil at the end sensitive, use shielded audio wire and ground the shield on the duino end only. (Make sure the shield on the other end doesn't touch the sensor, or it won't work.
Paul