Sunday I went to the hardware store, asked some questions, bought some parts, went home, and shut off the water. Tore things apart. Initial plan was no good; the antique tubing that fed the bad joint didn't work with any standard fittings. Back to the hardware store, design Contingency One, home again, failure, back to the hardware store, designed Contingencies Two and Three (because the store was going to close), failed twice more, hardware store closed, started to melt down.
Dementia reminded me that Home Depot was still open, and WalMart (cursed be the name) was open 24 hours. Off to Home Depot, design Contingencies Five and Six, home again. Contingency Five gets the water back on, but we now have one drop every four seconds. Contingency Six (which is Contingency Five with Gorilla Glue as a sealant) solves the issue. The non-standard tubing now connects to a standard shut-off valve, with a little help from six inches of polypropylene tubing, four hose clamps, one four inch standard 1/2" iron coupling, and Gorilla Glue. NOT code, but that won't matter unless we try to sell the house.
(For the record, I had a plumber look at this problem several years ago, and he refused to touch it; said there was no good way to fix it that he could guarantee wouldn't leak. There is NO code-worthy solution, but I will likely come up with a less obvious kluge that an inspector won't notice.)
I HATE home repair...