Thanks to Dale, I'm suddenly aware of the National Historical Geographic Information System, from the University of Minnesota. NHGIS hosts census data back to 1790! (An admission of mindlessness: I pulled up 1790 as a test and for a moment thought they only had partial census data, since Michigan wasn't listed. D'oh.)

Now I just need to restrain myself to looking up only the data I actually need, and not geeking out into semirandom data dumps.