I don't believe this exists.
I haven't checked, but I also don't believe you can access the Publishers used by TF, as it's an internal piece of infrastructure.
Thinking about it I'm also not sure this would really "work": TF not only cares about the current state, but also about transforms in the past. How would a current client get values from the past if the broadcaster hasn't actually computed nor broadcast them?
You could of course reason that for special clients aware of this limitation, this could be handled by some custom code, but that would make the whole setup rather brittle (only need one non-adapted client to start failing, need to update all clients, etc).
And something to realise:
If I was just publishing into a topic I could use the method getNumSubscribers().
That would only tell you there is some entity subscribing to the topic, not whether they are actually processing the messages.
Assuming reception == processing is not a valid thing to do and you have no way of knowing for sure. That's one of the characteristics of publish-subscribe.
