Overwriting yoast’s og:meta output?

Use the wpseo_opengraph_title filter:

function wpse_187763_wpseo_opengraph_title( $title ) {
    if ( is_singular() && $post = get_queried_object() ) {
        if ( $_title = get_post_meta( $post->ID, 'custom_field_key', true ) )
            $title = $_title; // Override title with custom meta title
    }

    return $title;
}

add_filter( 'wpseo_opengraph_title', 'wpse_187763_wpseo_opengraph_title' );

Update: The single = is intended, since we’re assigning the value & testing the expression at the same time. In other words, it’s the same as:

$_title = get_post_meta( $post->ID, 'custom_field_key', true );
if ( $_title )
    $title = $_title;