How to encode a large number of digital input signals into a signal result
You can easily encode 6 digital lines into one analogue line. I remember my old Sony MD walkman used to do something like this to encode all of its buttons into a single line. Each button connects a different value resistor to the sensing line. The resistor values are chosen such that every combination of buttons creates a unique parallel resistance. This resistance is measured, and the combination of buttons can be worked out by the microcontroller.



Here I have drawn the metal plate, and six wires hovering over it. Each wire is connected to a different value of resistor. What you are creating here is basically a manual Digital To Analogue Converter. 

Some examples:


No wires touching -> Output Voltage = Vcc
First wire touches -> Output Voltage = Vcc x 1/2 
Second wire touches -> Output Voltage = Vcc x 2/3
First and last wires touch -> Output Voltage = Vcc x 0.667/1.667