I don't see what protection a fight plan can offer in these circumstances.
I'm not sure how your ATC handles it, but over here prior to vacating controlled airspace ATC advise you of any IFR traffic affecting you out side of controlled airspace and prior to departure from an uncontrolled airport ATC (on an area ATC freq) advise you of IFR traffic.
So far as protection goes in NZ, it's quite simple really, everyone flying IFR needs to be on an IFR plan in order for ATC to be able to provide traffic information.