I don't have first-hand knowledge of either of the routes being bruited about, but I'm curious as to what other search methods either of you (surlyjohn or bicyclerider) are using.
Have you tried google maps' bicycle routing?
Have you checked with the state transportation departments?
It appears there aren't any (many) Nashville or Birmingham cyclists on this board. I'll claim some knowledge of viable routes for myself within about 30 miles of home, and a few, spotty, areas where I've lived, cycled, or toured. So, have you contacted bicycle clubs along the holes in your routes? I'm thinking of cycling clubs around McMinnville, Murphreesboro, Nashville, Franklin, or Columbia, TN; or Anniston, Birmingham, Tuscaloosa, AL and Meridian, MS, for the southern (Silver Comet) route. Have you checked out all the supported rides any of these clubs do? Is there a way you could link up some routes, with perhaps a few miles' cycling along unknown roads?
I'd be cautious about recommending some routes I ride for cyclists of unknown capabilities. If you could tell me about what you ride, I might volunteer more information. If/when you ask for help, can you tell someone what your comfort level is? Do you require bike-only, no motorized vehicle paths; very low traffic routes with limited curves and hills; or are you comfortable with high-speed traffic and decent shoulders; or even urban cycling? If I asked you, what's the highest traffic volume you've ridden comfortably in, can you answer? (Hint - do you need to pick one local road, and call your county roads department to find out what the traffic volume is on that road?) I'd think a local bike club would be more forthcoming if you included this kind of information in your request.
Finally, I applaud what you're trying to do. There are posters on these boards who pick their own routes, as you're trying to do, but they usually have a couple of solid tours under their belts before trying it. I think it's great that you're basically trying to replicate the work Adventure Cycling does on a semi-commercial basis, if even for your own use, without that much background. Do keep us in the loop about your planning, and how it goes when you get on the road.