Many thanks for your message.
I didn't see that the multiplayer plugin supports arbitrary synchronous messages between peers ...
I didn't mean instantaneous, but synchronous -- this means that the client send a request to C3 to obtain some information, and then waits until the respond from C3 arrives back.
If, possible, the client could also have a timeout mechanims - i.e. to not wait indefinitely.
But, surely, since all this happens over the network there is latency -- i.e. it takes time.
In theory, perhaps this can be accomplished by introducing a REST API plugin that treats the game as if it were a source of data on the web.
Perhaps, the NW.js is a good example -- quite like the NW.js plugin which augments C3 with file system capabilities, a REST.js plugin would turn C3 into a web data source.