Sending Data to Specific Clients with Subscriptions

[kaptcha0]

I've seen #3119, and getting the token via query parameters, the initial WebSocket upgrade request, or the first message sent. What I'm interested in, however, is sending data to a specific user via subscriptions, a way of identifying connected users, then sending User A data that is relevant to User A.

A use case of this might be a leaderboard of User A's friends only. Assume User A is friends with User B, but not User Z. A would want real-time updates about B, but not Z's scores, as they aren't requested, and may violate Z's privacy. The only way I would think of this to be implemented is like so:

public class Subscription
{
    [Subscribe]
    [Authorize]
    public User? LeaderboardUpdated([AuthenticatedUser] User currentUser, [EventMessage] User updatedUser) {
        if (currentUser.Friend.Any(friend => friend.ID == updatedUser.ID))
            return null;

        return updatedUser;
    }
}

[Authorize]
public async Book UpdateBook(User updatedUser, [Service] ITopicEventSender sender)
{
    await sender.SendAsync(nameof(Subscription.LeaderboardUpdated, book);

    // Omitted code for brevity
}

User being a class with User.ID being a string and User.Friends being an IEnumerable<User>. If the check fails in this situation, would null, an empty string, I'm assuming, be sent to User Z, since User B isn't one of their friends, or would HC just ignore it?

If there is a better way, or something built into HC, please let me know, as I had to use SignalR for this specific part of a project, even though the rest of it is using HC.