One of the more interesting things about xar is the representation of the the Table of Contents (toc) in XML, with the ability to store "subdocs", or additional XML trees, within the toc. You can easily add your own metadata to a xar file, in addition to its support for a variety of new metadata, including Extended-Attributes, both POSIX and Mac OS X's.
Anyway, I've been playing with prototyping a package manager based on XSL transformations of xar files. Basically registering extension functions that can perform the necessary actions on the xar file as it is transformed. It's a very interesting concept, and one I'm looking forward to exploring in greater detail.
I'll write some more about this as I have time to play with it.
I've been exploring implementing pyxar with the ctypes FFI module for Python, which is included in Python2.5. Currently, pyxar is a compiled C module for Python built with Pyrex. Ultimately, ctypes might be easier to maintain than Pyrex, and it certainly requires less on the user's end, now that it is included with Python.
I'm hoping to have something working soon.