I've been wondering if they may put this in one day.
I doubt i'd be called Diplomacy and the missions would probably involve covertly subverting Federation Colonies, sabotaging them, convincing them to "defect" to the Klingon Empire, convincing alien races to side with you over the Federation etc.
But essentially the concept would be the same.
Non-combat missions which give you rank points.
And when you max it out, you can have a "Federation Defector" BOFF, much like the KDF Exchange Officer we recieve for maxing our Diplomatic Rank.
I haven't thought what Federation species I'd claim if this became a reality.
But I'll cross that bridge if it ever comes around.