I fully agree, it would be a very daunting task, especially for one person. However, Forums like this though can certainly help. If we had an email address of the Donelson historian, or even to a local Round Table interested in Donelson, any one like you who is researching a specific regiment can then send in their own list of casualties, which would then go onto a central list.
Or perhaps a new Message Board could be created here, the KIA Message Board, with headings for specific battles. Folks could then go to that Board, find the battle heading, and list the names, regiments and companies of their "lost" dead killed in that specific battle. At least it might offer the beginning of the compiling of a list for each battle for serious researchers to examine. Eventually a fairly accurate list of the Unknowns might then emerge.
The men who gave their all deserve to be remembered, even if their exact graves are not known. Those in mass graves, or in a grave marked Unknown can at least be listed as KIA but grave not found, a list that can be added to the list of Known Burials.
Does anyone have a name and email address of the official Donelson park historian?