This is a good example of something that needs to be put away for whatever period of time may be necessary until perhaps something comes along that appears to be a decent solution. Rushing into a quick fix will be expensive.
I surely wouldn't get in any hurry about a fix. Just put the gun away for awhile, maybe show it now and then as an example of what can happen. Move on to something else and let this one just be forgotten for a while.
This is great advice. I once did a complete restoration on a 12 ga. damascus gun, which included a complete barrel refinish. A week or so later I noted a "leak" of some fluid into the left bore, 7 in. from the breech. I was devastated - as in your case, weeks and weeks of my hard labor had produced a beautiful gun that I was proud of, but within days of completion it was all negated by the discovery of an unsound barrel (which measured very favorably in wall thickness, by the way). I considered all the options, and found none palatable. So, I took the advice that John gave, and just put the gun up, and waited for a solution to present itself. In the meantime, I had a friend who became pretty proficient at re-laying ribs, so after a while, we stripped the ribs off of this gun. I expected to find rust and corrosion running rampant underneath the ribs. Instead, we discovered a single pinhole with no corrosion, that had probably been there since it was manufactured. The only reason it was discovered was because of the refinishing process. To repair, we gas-welded the pinhole closed, then re-layed the ribs. I refinished them (again), and have shot the living crap out of that gun ever since. The repair cost me practically nothing except a little in materials.
You may not be quite as lucky as me, but I still think it's a great idea to wait it out until a solution presents itself. In the meantime, I like the idea of 28 ga. tubes as a temporary solution the best. Good luck!