Show a username for unavailable appointment schedule time

On the appointments page, when it shows available time I want to show the username of the last client that have an appointment on the unavailable time segments, but I want to show only one username (last client) and only for admin users. Is it possible to customize this? The "Show Scheduled Users" add-on shows last 3 clients and it their usernames are visible for all other clients, I don't want this.