You make a good point. The O.R. does not include personal, company, battalion or regimental records. What records that did survive, like Mississippi's Confederate service records, are only the summery records that made it back to the State of Mississippi officials, and from what I remember, were saved from destruction/capture by a single person who hid them away. The majority of the records still in the field with the units were lost. Remember the after war report by the U.S. authorities I posted months ago that mentioned the Confederate records were being lost faster than they could archive them? The majority of county records were purposely destroyed as fast as the Union troops could get to the county seats, along with the courthouse.
What records I've seen in the Mississippi archives do not include a physical description or identification record...height, color hair, color eyes, race,...etc. That information could be found in the medical records, or death records, if those are available, but I do not know.
If someone was serious about finding out a number, I guess a comparision to the 1860 census and the known service records, person by person could be done. Proving at least not one name compares with a known FMC and someone in a Confederate unit. Boy, that would take a lot of work.