[I've answered my own question below, and for people searching for this issue in the future, I've replaced the question with a summary of the issue.]
In iTerm v3.2.8-9, "auval -a" does not find any 3rd-party plugins.
It has the same issue with utilities, scripts, etc. that have this kind of code:
- AVAudioUnitManager.shared.manager().components(matching: componentDescription)
- AVAudioUnitManager.shared.manager().components(passingTest: { comp, stop in
- AudioComponentFindNext(comp, &componentDescription)
Apple's Terminal does not have this issue.