People will try to satisfy their desire for PvP one way or another.
Make nomads restricted to certain systems, and allow them to attack without RP.
If you dont like silently attacking nomads, stay out of those systems. There are more than enough other systems to choose from.
I'd rather have silently attacking nomads in nomad systems than people desperately trying to PvP in New York with crumby RP.
I think the fear that they will spill out of those systems is exagerated. People who break rules break rules in every other faction they are in. A nomad out of his ZOI is the most easy to report and prove violation there is. If you are still fearful, I wonder if a powercore nerf can be made to depend in what system the person using the ID is in.