First, there's probably a pressure relief system already on the tank.

The problem is that is analogous to drilling a hole in a pressure cooker, it's either going to rupture the tank entirely, or blast the contents into the air.

Also curious where you would get a "quadcopter with a small drilling machine" with a couple hours' notice; even if you had one, it would be hard to maintain a position at one exact spot, and much of the lift energy would be expended in the torque reaction of the drill.