Well, sorry to jump into the middle of the discussion so late but here is my $0.02.
I drilled the hole couple of winters ago and so far no problems. The hole needs to be drilled in the lowest point of the pump housing. I drilled it while the pump was removed but you can do it with the pump installed. For that, you need to remove the plastic cover under the front bumper and in front of the driver wheel you'll see the bottom of the pump.
Having drains at the bottom of fans or ventilators, the "pump" is a fan as it pumps air not a liquid, especially if condensate may be present is a standard practice. Why it was not engineered in by Volvo engineers can be explained by lack of imagination, time, etc. There are poorly engineered systems on best of cars all the time.
Good luck.