@eschwartz: Then it probably didn't have a valid cover in the metadata, the hack handles PDOCs fine

.
IIRC, Calibre's default cdetype (after conversion, at least) is EBOK, you only end up with a PDOC if the book you're converting is tagged with the same tag as the one specified in the pdoc tag MOBI output setting.
The whole cover view thing (both cover & personal banner), while related to the cdetype in some aspects, is a bit more complex, but irrelevant for the hack anyway.