Choice of sensor for detecting touch (on/off)
I would be thinking in terms of running a grid of some very fine wires stretched tightly across the inside of the outer glass &mdash; single strands of AWG40 spaced every 3-4 inches, horizontally and vertically, would be nearly invisible.

Scan the wires individually for increased capacitance; if the wires that cross in your "target area" show increased capacitance at the same time, trigger your event. Such a system would allow the creation of multiple "hot spots" of various sizes, each triggering its own event.