Now available: RSSFilter, an open source Python module for modifying RSS files and blogBrowser-format RSS archives in place. It builds on XMLFilter. (Speaking of which, thanks to Mark Pilgrim for its recent mention in his b-links.)
The module can also be used an RSS parser for valid XML feeds, though it trades in ultra-liberal parsing for its ability to safely modify files.
Operations such as inserting, modifying, or deleting a post are designed to cause minimal disruption to the rest of the file.