I like the idea of adding a seal or o-ring. Never heard of that addition. And it is just some rubber so easy enough to drill out if it does not work. If you had to you could make one by shooting some silicone caulk into the hole and let it cure. Oil the metal first so it does not stickNo I stole it from my neighbour Not all good ideas are yours
You might look into "hot tapping" methods used for plumbing. Special tools and methods to drill into pressurized pipes. to add a connection.