I like the sound of that. To effect the diplomacy side of the game in such a way would be awesome and add a new element of deeper play.
Something else comes to mind. Might there be a way to use the spy/dipl boxes to effect say, supply or some other use.
Example. A unit that would be a spy/dipl, change it to a supply train that when traveling with the army would supply that army for as long as it lasted. Using the age factor to effect the life of the supply unit, which would be short and unpredictable.
It won't effect the stacking level of the army, but I have no idea if a slot for spy/dipl can effect anything in any way when a unit stacks with an army. But if it could, there lies some interesting possibilities.If not, never mind.