Hi Hugh -
your posting reminded me that my gg-grandpa Alfred L. Simmons, Co. I, 7th Ark. Inf. CSA was stationed at Hickman/Columbus KY during the timeframe you described (see URL given below).
Family lore claims that his brother (Peter Owen Simmons from Thomasville, Oregon Co. MO) and brother-in-law (William Bailey from Fulton Co. AR) went to pick him up and bring him home where he died, but I have been unable to locate any additional information. Wonder if he might be buried in one of the mass burial sites you describe...???
My Simmons kin moved to Fulton Co. AR from Walton Co. FL (of all places) after 1850 and before the 1860 census. Have any Florida or Arkansas kin?????