How about commander renaming? I know some do it and some don't, so it could possibly tip off who's who. (Oh, look, that player has named all his commanders Cainehill. I wonder who it is.) On the other hand Boron will go crazy if he can't rename his clam-carriers, as will I if the nation in question has random mages. Hence, I'd suggest allowing only certain kinds of renaming (preferable), or disallowing it completely (more reliable).
Here's a discussion about the possibilities of renaming even if direct communication is not allowed. Of course these would be cheating, but I'm more interested about renaming your mages: I like to rename them based on their random magics and empowerments and items (like, an earth Warlock empowered in blood, holding a clam and having cost a global would be named EBc+, to seek as complex an example as possible). Thus, if you encounter a player that doesn't rename his mages you'll know it's not me and will perhaps spare him owing to that, which would then be a decision influenced by diplomacy
Now, the options are either disregarding the matter, agreeing on a common renaming scheme, or turning renaming off altogether. Thoughts, anyone?