How can I use a static (ish) URL for the featured image for a post?

I figured it out. Using the Genesis framework, you can do this, adapted from http://dreamwhisperdesigns.com/genesis-tutorials/genesis-default-thumbnails/

/**
 * Default Image
 *
 * @author Jen Baumann
 * @link http://dreamwhisperdesigns.com/?p=429
 */
add_filter('genesis_get_image', 'dream_default_image', 10, 2);
function dream_default_image($output, $args) {
    global $post;
    $slug = $post->post_name;
    $thumbnail="http://foodiefun.imgix.net/featured/".$slug.'.jpg?fit=fill&bg=f9f9f9&w=400&h=600';

    switch($args['format']) {

        case 'html' :
            return '<img src="'.$thumbnail.'" class="alignleft post-image" alt="'. get_the_title($post->ID) .'" />';
            break;
        case 'url' :
            return $thumbnail;
            break;
        default :
            return $output;
            break;
    }
}