Instead of fixing it, I decided to go with Planet Planet instead. Well, sort of.
Sam Ruby has put a lot of work into a refactoring of the Planet Planet code, the result being Planet Venus. Among the most significant changes is the use of a cache directory containing Atom entries as individual files. This makes it easy to slurp the posts into WordPress, which has the advantage of giving easy access to historic posts, categories and users etc.
Prompted by a question on the planet development list I finally got around to putting together a plugin that would power a WordPress installation through the use of Planet Venus.
Installation and use of the WordPress Venus plugin (svn) is straight forward — dump it in the
wp-content/plugins folder, activate it from the plugins option screen, and go to the Venus option screen to configure its options: Path to Planet Venus cache directory, update interval and whether to link from posts back to their source.
Update: This plugin now lives in the WordPress Plugin Repository.