Update.
Saw an "Active Release Technique" practitioner. As I understand his diagnosis, the knee issue does not originate at the knee, but is a complex of other forces, including tight adductor, tight piriformis, tight iliacus, and non-activating glute medius.

Call me simple, but I'd sure like a diagnosis with known term, e.g., patellofemoral syndrome (though that doesn't seem to be this case), and pointed to one or more known programs of therapy, such as 'general knee mobility and strengthening program'. As with low back pain issues, sometimes it's better not to worry about the exact diagnosis, but just improve flexibility and strength around the problem, and that can improve/prevent the specific issue.

Any speculations or suggestions welcomed!