Turns out that our assumption that there was a busted supply line under the foundation was correct, and the plumber was able to pretty-easily pinpoint the issue to a hot-water line running under the slab from the hot water heater to the kitchen fixtures on the other side of the house. Unfortunately, this is a 40 foot run of pipe, and there is no good way to determine exactly where the break occurred, so there’s no way to know where to start jack-hammering up the concrete to fix it.
Luckily, the plumber knew just what to do — he’s going to run a new hot-water supply line from the second-floor master bathroom (which is directly above the kitchen), down the wall and into the kitchen, as a replacement for the original pipe running under the slab. He’s already cut and capped the original pipe (so no more potential leaks), and will run the new line tomorrow, which should allow our contractors to get back to work pretty quickly.
Ultimately, we lost a couple days on the rehab, and the total cost of the fix will be about $600 (plus another $175 for the diagnosis); not a bad resolution for what I thought was going to be a much more difficult and expensive issue to diagnose and repair. I told my guys to take their time from here, as they were originally trying to get the rehab done before next weekend, but now it looks like we’ll be putting it on the market the weekend of September 17 instead.