I found the leak, it's done. But not without weird things:
1. The 1" PVC pipe I initially found that wasn't leaking turned out
to be part of the sprinkler irrigation system. The 1" PVC main was
directly underneath it, about 10" deeper.
2. Speedy Jim was right: the leak was at the coupling/adaper between
the 1" PVC pipe from the meter, to where it connects to the 3/4"
copper pipe that goes into the house. This coupling was a good 15
feet away from the shut-off valve.
3. Get this: the leak occurred in the PVC adapter, a 1" inch female
slip fitting that reduces to a 3/4" female NPT, which screws onto the
copper pipe. In the middle of the adapter, there's a 1/"-wide
hex-ring molded into the fitting. The leak was a hairline fracture
that spanned 3 flats of the hex-ring, but perfectly centered around
the circumference. Odd, that, I think.
Anyway, after LOTS of digging, tearing up landscape timbers and brick
patio, and a few swear words, the leak appears to be fixed.
Thanks heap for all replies,