Simlpe plugin coded to refactor the code posted on a WPSE anwser

CptInMainQueryByMeta.php

<?php 
/**
 * Plugin Name: Cpt In Main Query By Meta
 * Plugin URI: http://wordpress.stackexchange.com/questions/129236/
 * Description: Allow mixing a post type with another on main query, only if the second match some meta query args
 * Author: Giuseppe Mazzapica
 * Author URI: http://wordpress.stackexchange.com/users/35541/g-m
 */
 
 
 /*************************************************************************************************
  *
  *  To make the plugin works, it needs to set up (passing to class constructor)
  *  a CPT name and some meta query args (see codex.wordpress.org/Class_Reference/WP_Meta_Query)
  *  both can be setted via filters: 'cpt_by_meta_in_home' and 'cpt_by_meta_in_home_args'
  *  E.g.:
  *
  *
  *   add_action('init', function() {
  *
  *     add_filter( 'cpt_in_main_query_by_meta', function() {
  *	      // SET HERE THE CPT NAME
  *	      return 'my-cpt';
  *     });
  *   
  *     add_filter( 'cpt_in_main_query_by_meta_args', function() {
  * 	   // SET HERE THE META QUERY ARGS
  * 	   return array(
  *	 	     'key' => 'my_custom_key',
  *		     'value'   => 'value_your_are_looking_for',
  *		     'compare' => '='
  *	      );
  *     });
  *
  *  });
  *
  *
  *************************************************************************************************/
 

class CptInMainQueryByMeta {
	
	/**
	 * @var array $filtered helper variable that allow run filter only once
	 * @access private
	 */
	private $filtered = array();
	
	/**
	 * @var array $filters the sql fiter generated by WP_Meta_Query
	 * @access private
	 */
	private $filters = array();
	
	/**
	 * @var array $args the arguments for WP_Meta_Query
	 * @access private
	 */
	private $args = array();
	
	/**
	 * @var string $cpt the name of CPT to  be added on home page
	 * @access private
	 */
	private $cpt = '';
	
	
	/**
	 * Constructor, check the arguments and start the add-to-query process
	 *
	 * @param string $cpt the name of CPT to  be added on home page
	 * @param array $args the arguments for WP_Meta_Query
	 * @access public
	 */
	function __construct( $cpt = '', $args = array() ) {
		if ( ! empty($cpt) ) {
			$types = get_post_types( array( 'public' => TRUE ) );
			if ( in_array( $cpt, $types ) && ! empty( $args ) && is_array( $args ) ) {
				$this->cpt = $cpt;
				$this->args = $args;
				add_action('pre_get_posts', array( $this, 'filters' ) );
			}
		}
	}
	
	/**
	 * Add filter hooks on main query for home archive
	 * run on pre_get_posts
	 *
	 * @param WP_Query $query the query being runned
	 * @return NULL
	 * @access public
	 */
	function filters( WP_Query $query = null ) {
		if ( current_filter() !== 'pre_get_posts' ) return;
		if ( $query->is_main_query() && ! is_admin() && is_home() ) {
			add_filter( 'posts_where', array( $this, 'filter' ) );
  			add_filter( 'posts_join',  array( $this, 'filter' ) );
		}
	}
	
	
	/**
	 * Add filter hooks on main query for home archive
	 * run on pre_get_posts
	 *
	 * @param WP_Query $query the query being runned
	 * @return string the filtered SQL where or join clause
	 * @access public
	 */
	function filter( $sql = '' ) {
		$filter = current_filter();
		if ( ! in_array( $filter, array('posts_where', 'posts_join'), TRUE ) ) return;
		// run once per filter
		if ( isset( $this->filtered[$filter] ) ) return $sql;
		$this->filtered[$filter] = 1;
		$this->get_sql();
	  	if ( current_filter() === 'posts_where' && isset( $this->filters['where'] ) ) {
			return $this->where();
	  	}
	  	if ( current_filter() === 'posts_join' && isset( $this->filters['join'] ) ) {
			return $sql .= $this->filters['join'];
  		}
	}
	
	
	/**
	 * Return the where clause merging post type condition with clause generated by WP_Meta_Query
	 *
	 * @return string the SQL where clause
	 * @access private
	 */
	private function where() {
		global $wpdb;
		$where = "AND ($wpdb->posts.post_status = 'publish') ";
		$where .= "AND ( $wpdb->posts.post_type = 'post' OR ( ";
		$where .= $wpdb->prepare( "$wpdb->posts.post_type = %s", $this->cpt);
		$where .= $this->filters['where'] . ' ) )';
		$where .= " GROUP BY $wpdb->posts.ID ";
		return $where;
	}
	
	/**
	 * Generate where an joing clause using WP_Meta_Query and arguments passed to constructor
	 *
	 * @return string the SQL where clause
	 * @access private
	 */
	private function get_sql() {
		if ( empty( $this->filters ) ) {
			$meta_query = new WP_Meta_Query( array( $this->args ) );
			$this->filters = $meta_query->get_sql('post', $GLOBALS['wpdb']->posts, 'ID');
		}
	}
	
}


// Three, two, one, go!
add_action('wp_loaded', function() {
	$cpt = apply_filters( 'cpt_in_main_query_by_meta', '' );
	$args = apply_filters( 'cpt_in_main_query_by_meta_args', array() );
	if ( ! empty($cpt) && is_string($cpt) && ! empty($args) && is_array($args) ) {
		new CptInMainQueryByMeta($cpt, $args);
	}
}, 30 );