Accidentally just found the brick! (Did this in story mode! Don't know if it matters which mode or not.) On the last page of the gliders, click underneath the 'Whirlybird 512' and it should appear. Please + this so the rest of 'em can see this cool Easter egg!
 "Fortress Magnus"){kind=icon}
 "Learn to Fly 2"){kind=icon}
 "The Last Defense"){kind=icon}
 "Twin Cat Warrior"){kind=icon}
 "Protector IV"){kind=icon}