If they meant option 3 (which my reading of it was), the only way I could see to do it currently from the source code also means unfortunately replacing the default image of Calibre of "library.png" (see
help page for how to do this).
I know I saw at least one
open ticket a while ago for this though I recall it was requesting a more complex implementation allowing different covers for different genres (tags).
As an aside if they meant option 2, then the same "trick" applies of putting your own image named 'default_cover.png' into the custom images folder as per the help page link above. However that one is more "intended" behaviour.