[Plugin: Basic Google Maps Placemarks] Add a Rating to Placemarks

You can use the bgmp_get-map-placemarks-return filter to change the content of the info window programatically.

function addPlacemarkRatings( $placemarks )
{
	foreach( $placemarks as $p )
		$p[ 'details' ] .= '[marker-rating]';
}
add_action( 'bgmp_get-map-placemarks-return', 'addPlacemarkRatings' );

That would add a shortcode called [marker-rating] to the end of each info window. Then you’d need to create the shortcode and have it output the rating.