Thanks for the replies.
I hadn't forgotten about a sling but failed to mention it. Besides string things in the butt stock and "storearm," I'd carry extra shells on a web sleeve on the but stock and possibly the sling if I found a method/design I liked.
As for Foster versus BRI-type slugs, the Foster slugs loose velocity much much faster than the BRI slugs. If I ever ran into a four-legged people-eater on the trail (cougars, wolves, or bears) I really like the idea a fifty-caliber lead hourglass slamming into their boilerroom than a Foster slug.
Also, EnvironMetal - the folks who make @Hevi-Shot offer three 'Dead Coyote' buckshot loads (one each in 12 gauge 2 3/4", 3", and 3 1/2") loaded with "T" size buckshot, made from Hevi-Shot (not @Hevi-Steel). I wonder how versatile and effective such a load would be.
I'm still going to go with an 18 1/2" tube as the compactness is one of the features I'm striving for. A cylinder bore will indeed likely limit range compared to a modified choke, but for a "survival" piece 25-30 yards ought to be plenty. I've shot blue grouse with a .410 Contender and a 12 gauge Survivor ought to give at least twice that range given the larger shot-load and more pellets. This gun isn't to hunt with per se, but to serve as a companion tool on the trail or in camp.
Again thanks for the input. I was surprised with the MANY other projects others have had making their own "Survivor" NEF singleshots. Using the .45/70 or .50 S&W has also crossed my mind. Also reboring a .410/.45 Colt to accept a .454 Casull if it could handle the pressure.