I think limiting PK hurts RP more than it does PK. The OOC forces are strong when someone has tried to kill you multiple times, your character hates them and you have to sit back and watch them fight someone else without being able to lay a finger on them even if it goes against your RP.
Our playerbase is (mostly) made up of mature adults right? So why not have a soft rule in place that more than 1v1 in an organized fashion can be allowed only if:
You are at war
Your character has a reason to want them dead on a personal level (they killed them, a friend, robbed them etc...)
Your character needs the help to kill them
I realize OOC chat can play a part but that always will and a good thing for the game shouldn't be restricted due to something that can't be controlled. The only issue I see is if you end up with a team of people who all don't need help to kill someone and they're chasing them down together. In that situation while it is OOC and does restrict RP/PK I'd say IMMs would be right to step in. Also the playerbase can handle this somewhat as well, if you see someone being ganged you can help or if someone else jumps your target you can both end up driving them away or killing them for different reasons then continue your battle.