You might consider posting your reference to the unit named on the Mississippi message board to get more information. You mentioned citizens involved in action - maybe as a spur of the moment thing, but not as a well-considered decision. First, if you were not regularly enrolled, you ran the risk of being hung on the spot as a guerrilla. Second, if you had any plans of being issued rations, equipment, uniform, arms &c, you would have to be enrolled.
Another point to consider - at the end of the war some officers began recruiting black companies based on legislation passed by Congress on March 5, 1865. You ancestor may have remembered that period of time.