For anybody who's interested... I made an experimental calibre plugin version of mobi_unpack. You simply select a folder where you want to unpack to (or a permanent folder can be set in the configuration) and click "unpack". It also supports extracting PDFs from print replica ebooks and splitting KF8/MOBIs built by newer versions of Kindlegen.
Let me know if anyone runs into any trouble (and be sure to restart calibre after installing the plugin before configuring/using it).
By all means, unzip the file to inspect the code if you're into that sort of thing, but otherwise the zip file is all ready to install as a plugin in calibre. You'll need a relatively new(ish) version of calibre to use it (>= 0.8.18)
And unless there are massive
structural changes to the current mobi_unpack code, it should be a matter of simply dropping the latest mobi_unpack files into the plugin (like I just did with v0.38).
P.S. - I'll start my own thread for this if there's an interest.
Plugin updated to incorporate the latest version of mobi_unpack
(2/20/2012) Plugin updated to incorporate mobi_unpack's v0.40 bug-fixes.
(2/29/2012) Plugin updated to incorporate mobi_unpack's v0.41 bug-fixes.
(3/8/2012) I've created a thread for the calibre plugin version of mobi_unpack in the appropriate calibre subforum. You can find it here