I like that idea, though I think the frame will probably be fine without the brazed-on covers, because those frames use stout tubing.

Using one of those covers will require filing/sanding away all remnants of the original braze-on, so you just have the tube. The cover needs a near-perfect fit everywhere. Obviously all paint needs to be gone from the braze area. The cover doesn't need a high-temperature braze, it could be attached with soft silver solder and an air-propane torch, y'know the ones from the local hardware store that screw to the top of a little propane bottle. No oxygen needed other than ambient air.

The soft solder you want is tin-silver based, lead-free, and is stronger than lead-based solder. Though not as strong as brazing, we don't need the strength of brazing for this cap. It has a small amount of silver in it (typically 2%), unlike silver brazing filler (which is often erroneously called silver solder, though it is for brazing). You need the right flux, should be sold along with the solder. Example fram Ace Hardware, $12. Apply flux to the outside of the tube and the inside of the cap, where they touch. The solder flows and bonds at a very low temperature, so the main mistake beginners make is getting it too hot. Try it on a test piece of similar weight and thickness to see just how low a temperature it flows at. Heat the tube more than the braze-on. The cap will overheat quickly if you point the flame right at it. Only after the tube is getting close to soldering temperature should you let the flame point at the cap for a sec. Don't apply the solder to the work until the work is hot enough to suck the solder in by capillary action. The work should melt the solder, not the flame. Hold the part to keep it from moving while soldering. The tang of a file (or any such random piece of steel in your shop) can be laid on it to weigh it down, the other end of the file placed on some nearby surface so just the narrow tip is pushing down on the braze-on.

Buy the smallest amount of solder and flux that you can, unless you plan on doing some copper plumbing. Silver soft solder is useful for a few other things in the shop, but not too many. Soldering the ends of gear and brake cables comes to mind, if you're fancy. Me, I stopped soldering cable ends decades ago 'cuz I'm lazy, the crimp-on cable ends are good enough for me. But for a concours bike maybe. Soldered ends really are better than crimps, and when you get good at it it doesn't take long. But the parts have to be clean, free of oil/grease, and there's a risk of dripping flux on other things, like your toes if you wear sandals (don't do that!) or on the bike. I don't recall if the flux is corrosive if you leave drips on the bike, but you'll definitely not want it on rims or brake pads (if rim brake), or disk rotors, or tires.

Send us pics of the completed job if you do this.