Add featured image caption to web service post data

//In your theme's php folder for example:

add_filter( 'wpak_post_data', 'wpak_add_thumbnail_caption', 10, 3 );
function wpak_add_thumbnail_caption( $post_data, $post, $component ) {
	$thumbnail_id = get_post_thumbnail_id( $post->ID );
	if ( $thumbnail_id ) {
		$image_post = get_post( $thumbnail_id );
		if ( $image_post ) {
			if ( !empty( $post_data['thumbnail'] ) ) {
				$post_data['thumbnail']['caption'] = $image_post->post_excerpt;
			}
		}
	}
	return $post_data;
}

//Then you can use post.thumbnail.caption in single.html and archive.html templates.