Could you please elaborate on how using ZeroMQ would solve your problem? I have a surprisingly similar setup and I'm also worried that this is going to be a bottleneck.
The RPC servers register the tags and channels we distribute based on that data. Any ZeroMQ server can process and deliver any event to the end RPC server.
We'll probably do a detailed writeup when this is done. We plan on using this for all the infrastructure wiring when it's done.