With this plugin you can change the authors name and url within the post. This might be needed, if you have many guest posts. It is available in: G
Works as intended so far. Does not interrupt anything else like Feedburner. One minor flaw, though: if you do not enter a custom author link it will default to the original username's author URL. This can probably be worked around by having a "Guest Author" username, or by disabling author links in the byline in your theme.