I posed this question to John O’Nolan, a WordPress core developer, who forwarded my question to core committer Aaron Jorbin. O’Nolan responded by saying (I’m paraphrasing) that there isn’t any official documentation on the WXR standard, but that reverse engineering a WXR export should give me all the information I need.
O’Nolan also noted that data more complex than blog post content is likely best dealt with through a database conversion/migration, rather than an import. I tend to agree here.
I found these two links to be of some help in my effort to reverse engineer the WXR standard:
http://olalindberg.com/2008/write-your-own-wxr-file-to-migrate-custom-cms-to-wordpress/
The latter link specifically deal with attachments, something crucial for my current project.
I hope this helps anyone else who is curious about WXR.