Here is a proof of concept to entertain you until the next version of Calibre gets out.
Many thanks to user_none for figuring the specs out.
Just get the attachment, extract the content, and run the little proggy (Windows only).
Enter the facts, ISBN (optional), the number of locations (the Kindle will tell you
) and the desired number of pages.
Press 'Generate' then look for a file named unknown.apnx in the same location where you keep the executable.
That's it! Rename the file after your mobi/azw file, and transfer it on the Kindle (or in 'My Kindle Content' folder in documents for Kindle4PC).
Enjoy!