WordPress action for WordPress User Query filter on author of specific post type.

gistfile1.php

<?php

/**
 * User query post type
 * 
 * @param WP_User_Query $query
 * @see https://github.com/WordPress/WordPress/blob/3.6/wp-includes/user.php#L528
 */
function prefix_user_query_post_type( $query ) {
	global $wpdb;

	$post_type = $query->get( 'prefix_post_type' );

	if ( ! empty( $post_type ) ) {
		$query->query_where .= $wpdb->prepare( " AND $wpdb->users.ID IN ( SELECT DISTINCT post_author FROM $wpdb->posts WHERE post_type = %s )", $post_type );
	}
}

add_action( 'pre_user_query', 'prefix_user_query_post_type' );