I couldn't find instructions on how to do this, so I poked around the code and sorted it out.
Default Powerpress = the itunes:author field is drawn from Powerpress settings. This works great if you have a single author, but not so hot when the author varies.
My hack = the itunes:author field is taken from the WordPress author field. This allows you to set different authors for different podcast episodes.
Change = wp-content/plugins/powerpress/powerpress.php, line 712 to
echo "\t\t<itunes:author>" . the_author() . "</itunes:author>";
WARNING = it is likely that you will have to redo this hack every time that you upgrade the plugin.