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.

It seems like the issue wasn't the pressure so much, but the temperature. The contents were supposed to be stored at 50 degrees. The thermometer for the tank was reading 100 degrees, but nobody knew what the actual temperature was, because the thermometer's maximum reading was 100. So the challenge was to figure out how to cool things down (which would relieve the pressure in turn)

All process tanks I've seen have various cleanout hatches, drain valves, pressure relief blowoff valves... even your water heater has these. Not sure about this tank in particular and obviously there were no easy answers in this particular event, but hopefully lessons will be learned.