Since rpyc is symmetric, clients can pass callbacks for the server to call. The callbacks then get executed in the client process. That's probably the simplest and cleanest way to do what you want.
So your server needs to expose a new register_callback(client_callback)
method, which stores the callbacks in a list. Then, whenever new data is received, you simply invoke all stored callbacks.
You'd also need to handle the case where a client which had registed a callback has already disconnected. It should be as simple as adding the correct try/except around the callback call (though I don't remember what's the actual exception type you'd need to catch).