My diagnosis was just confirmed.

The nearest Isuzu dealer is 400ish miles away, so I took it to an "authorized Isuzu service center" here in Fairbanks.
The tech there states: "she's leakin' pretty good" (at the valve cover gasket).

The leak is nearly INVISIBLE as well. You really have to get in just the right position to even detect it. The stream is small, and only travels about 1.5 inches before it is burned off. The design of the valve cover seal is such that a small tear will become a major leak very quickly.

I advise all drivers who are experiencing excessive oil consumption, or who have suddenly lost 1+ quarts of oil to check their valve cover gaskets thoroughly. This is not the only problem with the motor, but at least it's something detectable and easy enough to repair.

The new gasket is on order and covered by warranty. YeeHaa.