滤器nav_menu_link_attributes:

add_filter( \'nav_menu_link_attributes\', \'wpse_100726_extra_atts\', 10, 3 );

function wpse_100726_extra_atts( $atts, $item, $args )
{
    // inspect $item, then …
    $atts[\'custom\'] = \'some value\';
    return $atts;
}

add_filter( \'walker_nav_menu_start_el\', function( $item ) {

    $parts = explode( \'>\', $item );
    $out   = array ();

    foreach ( $parts as $i => $part )
    {
        if ( 0 === strpos( $part, \'<a \' ) ) // a start
            $out[ $i ] = $part . \' data-foo="bar"\';
        else
            $out[ $i ] = $part;
    }

    return join( \'>\', $out );
});