I don't think touch screens are anything to do with bio-electricity. I don't think those mobile phones can read our minds yet.
The capacitive screens just work on capacitance, and for that, you basically need a body with a significant self capacitance which is dependant on the size of the conductive body - like a human body, a car body, or a wire to ground (mains earth).
I would imagine that a lightweight electrode on the screen connected to a lead to ground via a switch or relay near the electrode would probably work. Give the electrode a diameter of, say, 5 -10mm.
If you have access to the touch screen circuitry, use a switched wire from the 0 volts.
If it is something completely encased - like a mobile phone or tablet, you can use a flat conductive sheet in contact with the back of the unit as the ground.
Worth a try.
Richard