David Maynard over on another forum scratchbuilt a set of sagged box cars that he had up at York last year. He built the frames and floors and then soaked them in a wash tub with block support on the end and a weight in the middle. The rest of the car was built around the sagged frame and floor. Lots of clamps were needed to build the sagged roof.
I seem to recall an article a few years ago in Garden Railways (I think) that briefly mentioned this. If I remember correctly, it was an indoor layout and the author sagged an old Bachmann passenger car to use a station or a maintenance shed. I think he said he did it by putting it on bricks in an oven on a low setting for a few minutes to get the desired droop. I don't remember though if he weighed it down in the middle or not. I would think that this would be a quick and easy way to get the desired result so long as the proper precautions are taken. Maybe someone else remembers seeing this, or knows more about it since I haven't tried myself.