Sensing human touch or proximity to large metal pipe
What capacitive sensors do is usually just measuring the detuning (i.e. the change in resonant frequency) that happens when a finger / human becomes the opposite electrode of a metal surface, which is normally only opposing earth "far away". 

With earth being far away from the metal patch and the finger being relatively close and large, and thus "bringing ground closer to the metal patch", these changes in resonant frequency are usually easily detectable.

Now, if we wanted to use your tube as metal patch, that would certainly still work, but the fact that its resonant frequency might be very low (compared to what standard capsense controllers do) will probably be a show stopper here; also, even a whole human will not change the effective distance to the second electrode of that pipe-ground capacitor much. 

I'd go and try to use a simple Doppler radar module, mounted atop, instead. Approaching humans move; sense that.