Sensing touch through a thick surface?
If you really need to detect touch through a metal surface, you could possibly do it using 4 insulated Piezoelectric pickups. There is a technique for detecting touch that cross references the frequency detected by 4 equidistant piezo pickups. It's a technique called SAW or surface acoustic wave. It's normally used for touch screens that are 3 dimensional or have surfaces that are prone to damage or deformation. It can detect location of a touch point, velocity of a touch (It can distinguish a tap from a press, to emulate a mouse click for instance) and it can detect a dragging or moving point.

If you wanted you could have a very simplified version to implement a few basic buttons or tap sensors. Instead of doing all the annoying frequency calculations, you could just position a few piezoelectric pickups where you want the actual buttons, then use a threshold detection to determine which has been pressed (the one with the largest signal peak). They are quite affective as they don't pickup any airborne vibration, so you don't have to worry about any loud noises setting it off, although dropping it on the floor would probably do it.