I think that the "preview button" should be substituted by a live preview function or at least be optional in ACP ( admins of forum have the ability to enable this function) . This way the user can see instantly what he is typing and can change anything he wants in real time.
Probably this should be when posting a new message or replying to an existing one. I cannot think of something worth using in ACP when previewing BBcodes.
There was a MOD since the era of 3.0.x phpBB which can be found here
. I do not think it has been converted to an extension though.