A small update on my extractor approach.
I took Ben Smiths advice and had a look at Calibre.
Now you get a neat UI instead of a command line dialog with some random popup dialog.
Also the extractor is now a reusable library (as it was supposed to be anyway). The First screenshot shows integration from within Conquirere. The second screenshot shows what happens when the extractor is called via the dolphin action menu. This way you can move forward/backward the single files and search/extract info for each of them. See what will be inserted and decide if you really want it or use a different entry/search engine and try again.
The UI has still some rough edges, but now you can change the search parameters manually and also restrict the search by author, year and so on.
So a nice UI that could be added to all programs that want to fetch and add metadata for its resources to nepomuk.
While I start to like the UI, the plugin structure in the background isn't well designed to allow it to work with all kind of situations. I'm sure this will be sorted out soon too.
The metadata extractor is now in playground if you like to have a look