I've taken a look; they  don't do what I suggest, of course, but they do decrease the cost of the problem that my suggestion is intended to address.

The mode I picture would, in its default behavior, act like the current default "nested" mode. It would increase expressiveness chiefly by by enabling commenters to choose an unobtrusive way to post. With this option available, I imagine that there would be moderate community pressure on commenters to select the full-display mode only for comments intended to advance the general discussion (vs., say, making a minor correction to another commenter, a semi-private joke, etc.).

Words and ideas I offer here may be used freely and without attribution.