-1 to Delrio's idea. That's the epitome of nonsense. A bunch of different privileged rules that need a manual to remember.
For Rodney's idea, nations can already issue "bounties" as long as it has leadership. A member of the leadership receives the gold coins through Send Payment and then he puts up the bounty on the board. End of the story. However, if letter of the marque is to be introduced in the game, this is pretty much the way it should be implemented.
Personally, I would want it to be NPC-driven. For PCs, we have laws. I will suggest a law to explain it.