Cool, glad it worked for you!
I agree that some sort of progress indicator would be useful. I just wanted to get "something" out there to see what the interest was, how people wanted to approach the multiple format/selection issue etc.
Your point about the edit book details window also confirms why I did not invest a great deal more effort at this point beyond "proving it was possible". As this plugin just wires together and resuses a few bits of Calibre code there really isn't any technical reason why it couldn't be built natively into Calibre. It is entirely down to Kovid and whether he wants to make the functionality available from screens like the Edit Metadata and Bulk Metadata dialogs.