For the next v3.4 update, I have provided a way to implement this request.
In the Preferences/ Tags(1)/ artwork/ set which pictures are displayed/ Edit List
you can now tick "prefer folder artwork for album covers displayed in the main panel"
When enabled, you can set the primary picture for each track in an album as embedded and as long you have at least one folder artwork (cover.jpg, folder.jpg, etc) associated to the first track in the album, MB would use that instead of the primary picture. So the folder artwork will show in the main panels (eg. in the Albums and Tracks view), except for the track info panel and now playing artwork which will continue to show the track specific artwork as is the case already.