Print only parent categories of post in custom RSS feed

I’m still interested in hearing how this can be accomplished using the WordPress function the_category_rss(), but I did find another way to do this:

<?php $parentsonly = ""; 
  // loop through categories
  foreach((get_the_category()) as $category) {
    // exclude child categories
    if ($category->category_parent == 0) {
      $parentsonly = '<category domain="' . get_category_link($category->cat_ID) . '">' . $category->name . '</category>'; 
    } 
  }
echo $parentsonly; ?>

This returns each parent category in the proper RSS validated format, using <category> and domain for the category URL:

<category domain="http://www.yourdomain.com/category-archive/">My Category</category>

Hope this helps someone else!