Steve,
I just did this when re-laying the ribs on some barrels. For an openning of more than an 1/8", fitting a small piece of steel into the hole is typical. It's usually a small triangle. At least it is on the Parker I took apart and other American doubles I have. This Lefever barrel rib has that triangle silver brazed to the rib.
On smaller holes, simply tilt the barrels muzzle down about 10 degrees, play the torch on the area to get the heat right, and add solder. If you keep them level, the solder will flow into the barrel gap and you'll end up adding more and more solder without getting the end filled and built up.
Another way I filled the end during sleeving was to machine a hourglass shaped piece and solder that in.
