I fear alliance wide channels would be detrimental, as they could cement alliance blocks even more tightly than they are right now. Alliances that want to talk to each other already have options through guilds and religions; these are not perfects, but their imperfections are related to IC contingencies and are actually helpful to the feeling of the game.
A fully continent-wide channel however, with absolutely no way for anyone to hide anything posted there, would be interesting. It would be a great channel for RPs and might even foster additional conversation.
Care should be taken that this does not mean every noble is automatically added to everyone's contact list.