Aren't most center speakers aimed to rediate up towards the listeners, obviating the need for an above screen speaker?
It seems to me that most speakers are designed to point straight at the listener which would mean the ideal place would be the center of the screen (like a movie theater), but that's not practical for most systems. That leaves either above, or below as other limitations dictate. However, as you may be implying, just below on a stand pointing slightly up does seem to be the most natural.
Eleven years ago in my clients home theater we used two center channels.
One above and the other below the screen. We used Mc Intosh speakers with great results. The dialogue is more focused than with only center.
To this day it is still very impressive.
That's what I was curious about, I wondered if the two speakers would vertically image or if it might just get too diffuse. It's interesting to hear that it can work, I'd like to try it sometime.