WYSIWYG on custom meta boxes while disabling main editor?

I just had the exact same issue. You have to include the editor in the supports argument array to get custom mce boxes to show.

I solved the issue by hiding it with CSS:

function move_posteditor( $hook ) {
    if ( $hook == 'post.php' OR $hook == 'post-new.php' ) {             
        add_action( 'admin_print_footer_scripts', 'remove_edit_div' );
    }
}
add_action( 'admin_enqueue_scripts', 'move_posteditor', 10, 1 );

    function remove_edit_div() {
        global $post;
        $post_type=get_post_type( $post->ID );
            if ( ( $_GET[ 'post_type' ] ) == 'custom_pt_name'  || (  $post_type == 'custom_pt_name') ) {
            ?>


                <style>#postdivrich { display: none; }</style>

    <?php }