Many thanks to James Louis Bacon for directing me to Civil War Washington, which includes a number of Emancipation Petitions from the war years. In a quick search I found several African American Magruders/McGruders, and others who were owned by, freed by, or had been sold by Magruders / McGruders or McGregors.
Choose “Texts,” where all are searchable by key words. Be sure to search under all possible spellings. The number of documents is not tremendous, but all have been transcribed, and include personal descriptions, detail about how the petitioner acquired the services of each person, and, in some cases, family relationships. Some had been brought from to D.C. from Maryland or Virginia. Good luck in your quest!