I actually just tested this and if a plunderer is "hiding" in Team Status, you can form your own team and attack him. As long as you both are in Team Status, the plunderer can be attacked. Below is a Screen Shot of the result between an attacker and me, a ninja in "Wanted Status". Since I didn't have permission from the other player, I censored his name out of privacy. But, he did say that full 3v3 team battles can occur between plunderers and attackers, something I have not tested, but plan to.

This is to prove that both he and I were in "Team Status" when the attack occurred. Again, I censored his name, due to privacy.

This is the message I got from losing the plunder. I censored his name, but wanted to include what was said, so that you could see that we were in a fight. I also included the System Notice I got after losing to prove that this was an attack on a plunderer event, and not a free duel.
Hope this can clear up some issues with the plunder mechanics! ^^