wp-jv-post-reading-groups.php

// Replace this
function wp_jv_prg_add_rg_meta_box_head() {
	add_meta_box('wp_jv_prg_sectionid','WP JV Reading Groups','wp_jv_prg_add_rg_meta_box', 'post','side','high');
	add_meta_box('wp_jv_prg_sectionid','WP JV Reading Groups','wp_jv_prg_add_rg_meta_box', 'page','side','high');
}


// With this
function wp_jv_prg_add_rg_meta_box_head() {
	$post_types = wp_jv_prg_get_post_types();
  foreach( $post_types as $post_type ) {
	  add_meta_box('wp_jv_prg_sectionid','WP JV Reading Groups','wp_jv_prg_add_rg_meta_box', $post_type,'side','high');
  }
}

//Also add this to get all post type names (including page and post) and allow others to hook into $post_types
function wp_jv_prg_get_post_types() {
	$args = array(
   'public'   => true,
   '_builtin' => false
	);

	$output = 'names'; // names or objects, note names is the default
	$operator = 'and'; // 'and' or 'or'

	$post_types = get_post_types( $args, $output, $operator );
  $post_types = array_merge( $post_types, array( 'post' => 'post' ) );
  if ( has_filter( 'wp_jv_prg_post_types' ) ){
    $post_types = apply_filters( 'wp_jv_prg_post_types', $post_types );
  }
	return $post_types;
}
function wp_jv_prg_modify_post_types( $post_types ) {
  return $post_types;
}
add_filter( 'wp_jv_prg_post_types', 'wp_jv_prg_modify_post_types' );