Unfortunately, that choice wasn't in the drop-down menu this morning when I was late for a meeting at my day job.
But your interim approach would be to extend the approval system or invent a new approval system so it could apply to replies, not just root nodes? And update all thread display code to hide unapproved replies? And add approval status notifications and approve-this form elements to the display of all replies?
Heck, that's a lot more work than the longer-term features I've been planning to add.
Of course, part of the reason for that is that the approval system and the thread display is some of the least-well-factored and ugliest code on the site. ):
(Node title not updated until after posting just to double-check that the posting bug is indeed fixed.)
In reply to Re^2: Current posting ability limitations (interim)