Editable text files as attachments or something whatever.
The key is - if you’re watching a replay, you don’t see any smack talk. You don’t see what the players were thinking. It’s like watching a football game without any commentary.
If there were a general text file that both players could edit and look at, and ALSO player-specific password protected text files (unlockable by inputting a password), a player that watched a replay could see:
The general text file, containing running in game comments like “ya i blew up ur battleship hawhaw”, and also password protected text, like “I built 2 transports to threaten invasion of London and attack Karelia”.
If the player entering the text were identified, and the password protected text identified by color (say blue for player 1, green for player 2, red for player 3, etc. etc.), it would make for a far more interesting replay.