Maybe not for other brands, but Panerai did a few exibition for the Marina Militare and their prototype for demonstration really did have it! If you manage to buy a Panny and it comes with the book "Panerai Historia from the depths of the sea" by Giampeiro Negretti ( or borrow from a local Panerisiti who has the book), look at page 35, this is official actual photo from Panerai!
And the caption says "A 1938 prototype made for demonstration purposes; the see-through back allows the movement to be seen"
Yes, while i know they can just open it, but hey dont point fingers at me, i wasnt even alive back then! So when Panerai chose to do this, it was probably unique in their time, since you said that no one else does it.
Cheers
Stephen