The good news is that I'm about 50% done with a GUI plugin that will let you import nearly any simple wikidata property relationship you want into nearly any column (custom or not) and any IDs you want without making any changes to the plugin -- just preferences changes.
The bad news is that I got so badly sidetracked in the QT MVC model that the plugin is only 50% done and I may not have a lot of time to work on it in the next week.
But when it is done, it'll be pretty and extremely flexible.
The end product will use this plugin to handle assigning wikidata IDs to books and formatting any IDs imported by either plugin and importing covers. The other plugin will handle importing all other possible metadata (with some overlap), but it would be preferable to import it with the GUI plugin because it can merge data where this metadata plugin can only overwrite it.