That's probably rather an idea for an enhancement in Calibre itself than for a plugin but maybe someone would be interested:
It would be nice to have the possibility to use icons/emblems in the book details (e.g. below the cover), just as in the case of columns and the grid view. This way one could save some space (e.g. a little icon to mark the book as read might be more optimal for some users than the text information), especially if they have quite many columns displayed in the book details and the view is quite clattered.
The request on Launchpad:
https://bugs.launchpad.net/calibre/+bug/1516186