Just some thoughts :
I think its going to depend on how is the temperature outside , how big is your house , etc. I live in europe and we dont have your problem but i saw video where police describe how they drive around scanning houses and look for like places where hot comes out where it shouldnt come.
600w is not much imo. But think if you had like attic with kws and the attic would have like a hole where very hot air comes out and the heat scanner would pick it up and the attic would clearly look as being hotter than the house , and you would live in a neighborhood known to have lot growers. And so on and so on.
If you have to heat your house , maybe you could use fans to spread it around the rooms and turn off the heating.
This is turning into a pretty uninformative post but my opinion is that if you think the scanning is a serious threat. Then you have to spread the heat evenly and make sure it is not going out from some single hole. I dont think the scanner really sees through walls so your cooling tube thing might work. But you gotta do some research on that , i think the main idea of my post is spreading the heat around the place.
Also its pretty dam hard imo to cool a room electrically. Lets say i have 3kws and i would like it to be cool without venting it out , then id propably have to spend lot of watts to get it happening.