The requirement of running the sub processes on different machines makes this a hard(TM) problem. If it was up to me, I would have the processes listening on more than one port with a different handler for the "control" port.

You would then need to connect this to a signal to ensure that you get a timely response from your control port and make your other processing reentrant so that interuptions (to handle control messages) do not disrupt normal processing.