I totally agree, it would be nice to have some sort of planning stage. I obviously don't know the details of how the lobby system works (or more directly, how much control you all have over it), but a possible solution that would be easy(ish) to implement would be to have a team only text box in the lobby.I like the courier system the way it is, it makes for great multiplayer fun. My one complaint would be the inability to form a battle plan.
With that said, my wish-list for the courier system would be the ability to type what I want freely in the message box. Obviously it wouldn't work for the AI, but it would be great for MP. Also, somebody stop me if this is actually in the game already, but the ability to send the same message to multiple people at the same time would be nice.
PS, this game is unbelievable!