How can I modify RSS item titles to be either the title or a custom meta field?

Something like this (not tested):

add_filter('the_title_rss', 'custom_feed_title');

function custom_feed_title($title) {

    $headline = get_post_meta(get_the_id(), 'headline', true);

    if (!empty($headline))
        return $headline;

    return $title;
}