I just succeeded in adding a rail button using a variation of K-Tesh's approach. For the forward rail button, I first made a block from several sheets of balsa (glued with perpendicular grains) and drilled a hole in it, a hair wider than the screws for my rail button. Next I epoxied the nut for my rail button over the balsa block. Lastly I put epoxy through the hole just above the forward centering ring and maneuvered the balsa block into the epoxy, with the nut between the balsa block and body tube. I made sure the bulk of the epoxy was on top of the centering ring, so the balsa block is glued to the centering ring (not the body tube.)
Getting the balsa block + nut into place isn't easy, but it helps to use a magnet on the outside of thin-wall body tube to maneuver the nut. Once it's close to the body tube hole, you can use a pin or toothpick to center it. I also used a yardstick to put pressure on the block + nut for a good epoxy bond. A small flashlight was essential to see what was going on inside the tube!
The aft rail button was easy to add: just drilled a hole aft of the bottom centering ring, and used a crescent wrench to hold the nut on the inside of the tube's open end while tightening the screw for the rail button.