Through a ridiculous number of scripts I have managed to make diplomacy possible! The method is a little(very) awkward but creating alliances and declaring war is now possible!
Each player has an Isildur in the cliffs near every other player. These Isildurs are what control the diplomacy.
To create an Alliance both you and the player you want to create an alliance with must use Hold Ground stance on the Isildur near eachother's bases. Once the alliance is created a horn sound will play.
To Declare War use Aggressive stance on your Isildur near an allies base. A horn and beacon will show when someone has declared war on you. You may not declare war if you have units inside the base of the player you intend to declare war upon. Nor will you be able to declare war for 90 seconds after making an alliance
This is a complicated map and required a lot of scripting, there are bound to be errors and bugs. Please report them here if you find any.
Note that vision is not immediately shared between allies. Also note that after either declaring war or making an alliance that your Isildur switches stance back to Battle Stance. This is intended and shows that the script has worked, do not change it back.