I would like to be able to have a file with U.S. Census data pre-aggregated at some level of the Military Grid Reference System (MGRS). Ideally at thier 10km precision level, but I'd take whatever I could get.
Unfortunately, I don't think this exists as a pre-aggregated data file.
However you could use a tool like census_area to get a close approximation. With this tool, you could download block or block group level data and aggregate them your desired geographies.
(I'm one of the authors of this library)