Include related episodes in the feed by getting the usual posts, reading their post meta value, and then re-running the query

feed.php

<?php

// pre_get_posts is hooked to run `include_the_fringe`

	public function include_the_fringe( $query ) {

		if ( $query->is_feed() && $query->is_main_query() && isset($_GET['fringe']) ) {
			
			$ids = $this->find_fringe_episodes($query);
			// $ids = json_decode('["490","491","381","245","219","168","132","130","126","122"]');

			$length = count($ids);
			if ($length < $query->get('posts_per_rss')) $length = $query->get('posts_per_rss');

			$query->set('posts_per_rss', $length);
			$query->set('post__in', $ids);
			$query->set('cat','');
			// $query->set('category_name', '');
			$query->set('tax_query', null);

		}

		return $query;

	}

	private function find_fringe_episodes($query) {

		$qu = clone $query;
		$qu->set('fields', 'ids');
		$posts = $qu->get_posts();
		$ids = array();

		foreach ($posts as $post_id) {
			$id = $post_id;
			$fringe_id = get_post_meta($id, 'nexus-fringe-episode', true); 
			$ids[] = $id;
			if (is_numeric($fringe_id)) $ids[] = $fringe_id;
		}

		wp_reset_query();
		wp_reset_postdata();

		return $ids;

	}	

?>