OP:
It depends on what kind of a CB system you're asking for. I think something along the lines of CK2 would be a horrible idea, (where you aren't able to attack until you have one). Something like EU3, however, where you can declare Wars of Aggression without a CB, at the cost of your good reputation, would be a good addition IMO, granted a whole infamy system would have to be included.
If one was to make the analogy that Napoleon didn't need a CB, and you were to apply it to an EU3 system, it would make good sense in respect of the game's mechanics that most of the wars against him that he didn't initiate were either a form of punishment, or nations trying to maintain the balance of power by cutting France down to size. Also, please check me on this, but I don't believe there were any wars between the other European powers amongst themselves during this period. I do like the novel idea of this actually happening in the game quite a lot.
From what I can tell, doing something like this would strip something away from the character of the game; that is, one giant Napoleonic brawl.