Create navigation links for a specific set of posts (post_type, meta_key, etc...).

wp_get_custom_post_nav.php

<?php
/**
 *  Create navigation links for a specific set of posts (post_type, meta_key, etc...)
 *
 *  @param   string  $post_type  Optional post_type
 *  @param   string  $meta_key   Optional meta_key
 *  @return  array   $nav_links  The array of navigation links
 */
function wp_get_custom_post_nav( $post_type='post', $meta_key='' ) {
    global $post;

    $args = array(
        'post_type'      => $post_type,
        'posts_per_page' => -1,
        'order'          => 'asc',
        'orderby'        => 'meta_value',
        'meta_key'       => $meta_key
    );

    if( ! $posts = get_posts( $args ) ) {
        return false;
    }
    
    $nav_links = array();
    $post_ids = array();

    foreach( $posts as $p ) {
        $post_ids[] += $p->ID;
    }

    $id = get_the_id();
    $current = array_search($id, $post_ids);

    if( ! empty( $class_ids[$current-1] ) ) {
        $prev_id = $class_ids[$current-1];
    }

    if( ! empty( $class_ids[$current+1] ) ) {
        $next_id = $class_ids[$current+1];
    }

    // Check for previous posts
    if( ! empty( $prev_id ) ) {
        $nav_links['prev'] = array(
            'link' => get_permalink( $prev_id ),
            'title' => get_the_title( $prev_id )
        );
    }

    // Check for next posts
    if( ! empty( $next_id ) ) {
        $nav_links['next'] = array(
            'link' => get_permalink( $next_id ),
            'title' => get_the_title( $next_id )
        );
    }

    return $nav_links;
}