While probably the best way to attempt it, you can't change macros while in combat, and if new players join in during the fight, or there are Rogues that you can't see from the start, then you're still going to be scrambling to fight them.
You could always turn on Repeater and try to type out /target CharacterName lines real quick (while you're getting beat on). /shrug
You're testing this on the floor (ground level) of the open world Stranglethorn PvP free-for-all arena? The moment you touch the arena floor everyone on that floor turns hostile toward one another. Like I said, I haven't been there in forever, and I've certainly never tried to multibox it, so anything could have changed (however it sounds like Multibocks already tried it and everyone is hostile).