Evernight, a web based fantasy wargame that I used to play (evernightgame.com) has a good system for diplomacy. Everything I know about NAP's I learned from them.
They have a good message log, which would be very helpful in DOM3 MP for many reasons beyond keeping people honest. There's also a setup where you can rate other players on a scale for various things like honor, communication, skill, etc. and type up a blurb about your experiences with them for other players to read.
Could be useful here if it were implementable.
-Ubercat