That is a great idea. I fully endorse it, and it would be a great way to control the flow of information (since we already enjoy playing "thought police" and making sure there is no body left behind). I also think the overseer should be able to assign expedition leaders, mayors or nobles to draft letters to other fortresses with certain predetermined messages, like "Request more migrants", "Request soldiers", "Send tribute" and more. This would allow you to curry favour with your home civ; sending them your riches would make them more willing to send an armed force for you to house, but killing your own caravans would make them more likely to send the armed force against you.
Even better, letters could show up in worldgen (probably written on hide or leather) with random names. The humans embroiled in The Battle of Sparks could send a dispatch to their enemies, the elves, asking for a peaceful settlement and offering goods ("The 567 Treaty of the Battle of Sparks", "The 567 Battle of Sparks Dispatch", etc.), or a goblin civ could demand food and slaves from a weak dwarven civ ("The 1050 Letter of Subjugation", etc.). Letters could be saved in keeps, and historical hostile letters could be saved and make that civ less likely to desire peace/trade with their historical oppressors (since the people in charge have read these old letters and were appalled). They could also be lost, either by wreck of cart or the sacking of a keep, or stolen by kobolds even. An adventurer might have to steal back the Elven Reinsurance Treaty of 938 from bandits or even terrorists before the sun sets in two days' time or the elves will declare war on their home civ.
This is a fantastic idea! Thank you for posting it; it never really occurred to me.