I just examined my Marlin Cowboy M1895. Without taking measurements, it appears that dovetail cut for holding the magazine tube mounting stud leaves the barrel wall VERY thin, probably 0.030" to0.050".
I'm not saying the barrel should not be milled for a dovetail, just that it requires extraordinary care to not leave the barrel wall too thin. The Marlin factory uses CNC controlled milling machines and fixtures to control this dimension. A manual machinist will have to be extremely careful.
That picture of the barrel band sling mounting stud is for a round barrel. It would have to be modified by milling or filing to fit your octagon barrel. I would try for for 100% contact on the bottom four faces of the octagon barrel, or close to 1/2 square inch of contact. Careful sweat soldering will produce a plenty strong attachment for your sling swivel stud.
I would file, fit, and scrape the contact surfaces of the stud and barrel to a blue-free light-tight fit and tin both surfaces using rosin-core solder. Flux again, clamp in place, and apply heat to sweat the joint. Use solvent and brass brush to remove excess flux. Correctly done, the barrel bluing is not affected.