I thought the EAW diplomacy was elegant and well-done, mixing event choices with historical tendencies. A Napoleonic Campaign from 1805-1815 would be challenging, since there are more permutations and it was common for major powers spending time fighting on either side of the two main groupings, the French Empire and its allies and the British-funded Coalitions. Here are some thoughts on how the EAW system can be adapted to work with NCP2 PBEM campaign game.
First off, this will assume a two-player game, with one person controlling France and its allies and the other Britain and its Coalition. I know many like the idea of multi-player including many more positions, but for now let's assume a one versus one.
THE DIPLOMACY BOARD
As in EAW, actions are taken and allegiances tracked on a graphical board, which includes information on the various tendencies of the power represented. However, to allow for the greater length of NCP2 and added twists and turns, the "board" itself can change as the game advances based on in-game occurences (like a power surrendering or asking terms) or through player choices (e.g. the French instituting the Continental system or Britain imposing a strict blockade). Sometimes the board will change due to minor actions, such as expenditure on a political option to create the Grand Duchy of Warsaw, and other times a wholesale change if a Coalition itself breaks up (e.g. as happened after Austerlitz). So, unlike EAW, the NCP2 diplomacy board will be ever-shifting and at times will completely reset all or in part
In addition to sending Ambassadors a la EAW, the game should also add Spies. This will be coupled with a Fog of War, whereby what is visible on the Diplomacy Board may include incomplete, no, or confusing information. Placed on one of the spaces for a power, a Spy can be given different missions, either to mislead (add to Fog of War) or gather information (reduce Fog of War)
Trade can be automated, such as how EAW handles it when a power is neutral. However, players should also be able to send aid to a particular nation
Just some thoughts. Maybe other games out there like Empires in Arms have other techniques that can be adapted. The idea is to keep it simple and understandable, as well as influenced by actual history.