when you refer to "header", to me that means the header on the Album and Tracks layout. As i said its implemented and works fine
For the artwork only layouts like "Artists" or "Albums" you can double click on the picture or the text below the picture. How MB behaves depends on settings for the artwork layout ie. it drills down to that artist/album or it plays that artist/album.
If you are refering to the expanded panel ie. click on an album to see all tracks, then yes you are right - double click doesnt do anything