I doubt that calibre can handle automatically doing that by itself. You could write a script to do it though, the way I'd set up the output would be one txt file per thread, with folders made for the categories. It'll take a little effort to get it the way you want it, but if you have a little coding knowledge, it shouldn't be much trouble.
Edit: If you do make this, you can probably generalize it (it may already be generalized) for any phpBB based forum. That'd be a pretty cool tool, tell us how it goes.
One more edit: Here's a starting place for including this in calibre, the script'll have to be python based:
http://calibre-ebook.com/user_manual...ml#term-recipe