This would be very helpful, and the proposed solution seems simplest. However, I'd suggest for consideration an alternate implementation:
As proposed, the fix would provide the option to automate the duplication of a post for multiple groups. However, in this implementation, the teacher would then show multiple copies of each discussion on his/her screen. In the case of only a few groups and only a few discussions within a forum (in this example, Q&A), this wouldn't be unmanageable. But in a case such as discussed here (https://moodle.org/mod/forum/discuss.php?d=172853#p850072) (30 separate groups!) and/or with multiple discussions within a forum, the screen for the teacher could quickly become very unwieldy.
I'd love to see a sort of "all groups" option that created a single discussion, but tagged each contribution (after the instructor's open) by group. A student then viewing that single discussion would see only discussions tagged with his/her own group tags. Ideally the instructor would have a single discussion link with a drop-down box (or radio controls, or list with checkboxes?) to "filter" the content of the discussion by group, essentially simulating the view of a student within that group).
Just a thought. Our staff is definitely looking forward to any implementation of this feature.