In this suggestion, you want to leave a message (be it in high humours or low) for someone offline, maybe in another timezone, to read.
The details may vary, but the outline is below.
1) Parchment can be inscribed with a fixed number of characters e.g. 160. Once written, the message can not be erased.
2) The piece of parchment can be tied to an arrow, maybe with string, and then shot wherever you want (probably a rather off bounds location, e.g. into someones claimed territory or across the frontline in a siege).
3) The message arrow may then be deconstructed into a readable parchment and an arrow.
4) Possible problem: what are the current mechanics for the deletion of items "strewn around" outside containers - to fulfill the purpose, the message arrow should be somewhat resistant to item "resets".