We do have a MonClient instance.  But I can imagine a scenario in which a client would like to keep his MonClient instance connected to its current mon, and ping another monitor just for the kicks of it (or to establish there's another monitor available).

In a nutshell, using the current MonClient is not a blocker.  We can easily do it.  A bit of adjustment on MonClient::ping_monitor() and we'll be golden.  But I wonder if we do want to mix the ping thing with the current monclient used by RadosClient, even if as of this patch that's the only way it is used.