I checked with the team and this is what I was told:
We currently do not encapsulate published art. This includes prelims. The Campbell one, consisting of several partial sketches on a piece of paper, is far enough removed/dissimilar from the finished art to allow it. As for the Liefeld ones, they are generally complete and broken down into the panels used in the finished published book. I'm not familiar with the Capullo ones mentioned, they may have been before my time or like the Campbell one, far enough removed from the published work to allow it. We are planning on offering a service for the grading of published art in the future, though currently there is no set time frame for this service.