1. A process control system operating to control a process within a process plant, the system comprising: a process control equipment resource; a process controller; a plurality of user interface devices including an operator user interface device and a maintenance user interface device, each of the plurality of user interface devices including a processor and a display; a data structure storing an assignment record for identifying an assigned user interface device for the process control equipment resource, the assigned user interface device for the process control equipment resource being one of the operator user interface device or the maintenance user interface device; and a message handling routine that, when executed, causes the process controller to to selectively forward or block messages to the process control equipment resource based on (i) which of the plurality of user interface devices is sending the messages and (ii) which of the plurality of user interface devices the assignment record indicates is the assigned user device for the process control equipment resource, such that a message sent to the process control equipment resource is blocked: (a) when the message is sent from the operator user interface device while the assigned user interface device is the maintenance user interface device; and (b) when the message is sent from the maintenance user interface device while the assigned user interface device is the operator user interface device.