Change Default Artwork in iTunes 11
They seem to have found a fix, but for Windows. I haven't found a similar one for Mac, I'll update this answer if I find it.

Note: The above only applies to changing the default artwork for viewing by genre, not by artist, which is still the unanswered question.

While it is possible to change this by playing with ratings, there doesn't seem to be a manual workaround, as existed in 10.7.