I don't know your answer, but I'll just ask the obvious...

There IS a return spring on the linkage as well, I assume? In addition to the rotary return spring on the throttle body, that is? I've seen Z's where that's been discarded.

Yes, in addition to the rotating spring on the throttle body, there is an extension spring on the linkage located up near the throttle body. It's rusty and should probably be replaced, but it is there. There is also another spring right at the gas pedal.

The weird thing about this sticky throttle body is that it is perfectly smooth and easy to rotate when there is no vacuum behind the butterfly, but when there's high engine vacuum at idle, it gets stuck a little above idle. Kill the motor, and it shuts to the stop screw.

Wrapping that throttle body return spring around an additional turn might prevent the issue, but I don't think weak springs are the root of the problem...

Is there any play in the butterfly valve shaft? I wonder whether it's turning in an oval hole.

Couple thousandths. Nothing that I would consider problematic (if it weren't for the fact that I'm having a problem). :paranoid:

The shaft is chrome plated for wear resistance, and the aluminum throttle body has steel sleeves in it where the shaft enters and exits.

This is annoying... It's gonna be Z season soon and I gotta get to the bottom of this.

Hmmmm... Rather than trying to find resistance with a vacuum, have you tried simply forcing the butterfly in different directions to see if you can reproduce the binding? You say there are no wear marks between the butterfly and throttle body, but I really wonder about that, especially since the binding is just off of idle.

You might try using a lapping compound (e.g. paint compound) to ease whatever is binding. Apply it sparingly around the throat of the throttle where the butterfly closes. Avoid getting any around the shaft. Then apply the vacuum and work the throttle. If your butterfly is binding somewhere, you'll see black aluminum filings there, AND you'll take off enough thickness of the metals to ease the binding if you keep working the butterfly. However, don't get the stuff in the shaft! When you're done, take it all apart, and clean it thoroughly to get any stray abrasive off/out of it.