Use of internal PCB ground plane as ESD baseplate/EMC reference plane of product in plastic enclosure
Based on what you have described, your device isn't internally referenced to earth, so the last thing you want to do is have the ESD event come in a connector wander around your PCB for a bit before exiting through another connector on its way to being sent to earth somewhere else. You're just making trouble for yourself. Put ESD protection on all signal/power lines as close as possible to the connectors, and routed to the connector grounds and you shouldn't have any further ESD concerns. 

As for the EMC side of thing, the metal reference plane you have shown appears to be there to deal with a residual common mode signal that comes through the cable. It is usually far better to try to eliminate such common mode signals first, before you start worrying about mitigation options. If you design your communications interfaces, connectors and cabling correctly in the first place, then you probably won't need to do anything else. With EMC prevention tends to be the better option if you have the luxury.