The spirit of the proposal is much more along the lines of homesteading. Give people in the developing world an extra opportunity to move to the United States—if they want to—by coming to an area of the country that it seems most Americans prefer to leave.
There are a number of urban and rural homesteading initiatives in the United States. For a number of reasons, they aren't effective. We've failed before starting because we frame the issue in terms of population. Finding people who are willing to live in urban Detroit will do little to address the crisis.
Furthermore, we needn't issue more Green Cards as bait. Yglesias correctly assumes the political impossibility of his policy prescription. Other cities, such as Schenectady (New York), already understand the path of least resistance. There are plenty of immigrants already residing within the United States who would find urban homesteading attractive if you cared to target that demographic. Follow the secondary migration out of New York City and see what is going on in Reading.
Finally, enough with the "leaving" myth. The acute outmigration will settle down in a few years as the national economy finds its footing. The challenge quickly shifts to one of an aging population and a lack of inmigration, pretty much what was going on before the Great Recession. Immigration is a good way to address the situation and Detroit is still a gateway city. Unlike Pittsburgh, Detroit already attracts immigrants. Lots of them. You might encourage more to play entrepreneur in the blighted neighborhoods close to downtown. Federal legislation isn't necessary.