Is a human (or other) electrical signal absolutely necessary to activate a capacitive touchscreen?
The reason the stylus worked even through layers of wool is related to the very nature of capacitance: It is not about conduction (except at the electrodes themselves), it is about the insulation between the plates: Wool is about as good a dielectric as dry air, if it is not woven with some metallic fiber.

The way capacitive touch screens typically work is that they sense the change in capacitance that occurs when an earthed body - essentially any large conducting body, such as a human - comes close(r) to the surface electrode. By using a metallic stylus, that "earthed body" is being brought closer, with merely the wool in between. 

This capacitance change can be measured in a number of ways. A common approach is to inject a high frequency signal between the sensing electrode and the device's own ground plane, with air (or glass, or some other material) as a dielectric between these two "electrodes". Any other conductor coming into proximity changes the dielectric (i.e. insulation) pattern, thus changing the capacitance, and thereby the pattern of load shown by the capacitor across the native capacitance.