Robin,
Thanks for the interesting and challenging question, looks like it may be feasible to create a new topic/channel and subscribe to it, since we can not alter server side implementation, it may have all to be done client side, every client checks if topic exists or not it it does not exist it creates it and subscribes to it, since there are no SOQL queries in IBM BPM we may need to think how to inject the server notification logic, we can probably link with existing topics or notify server from the client itself to push to remaining clients. This needs some R&D i guess I will play with and post updates. IBM BPM may have an equivalent of SOQL queries, but there is no documentation of it if it exists. Class file digging may reveal something.
If you just want to subscribe to existing IBM BPM created topics that should be easy.