To answer the question from my girl Aeolla:
It was decided when Bree/Jake/Ruud declaring war on the MC so that he could bring his guardian NPCs there (despite an obvious IC alliance) that the tactic was 'cheating' and would be punishable. I made a post about it in leader forums which probably still exists. This took place in early/mid 2009, I believe.
As of now, a tribunal member can bring a NPC only into nations they are at war with and cabal headquarters they are at war with the cabal of.
I have LONG since recommended (since the creation of tribunals, actually, if you want to go back and look at those old posts

) that tribunals be allowed to bring their guards anywhere in the mud. Remove bounty hunting NPCs and let tribunals move their guard NPCs around anywhere they want.
You can only order tribunal NPCs based upon a combination of your charisma/level, and therefore the "restricted" system already exists.
So, a few points:
1) Let tribunals bring their guard NPCs anywhere.
2) Remove bounty hunters.
3) Let the strongest cabal by far in the mud (Harlequin) deal with it.
4) Leave the MC as they are, this will already be a substantial buff to them.