Via Facebook, a fascinating idea: Letter Larping:
Letter larping is, in short terms, just what the name implies: larping done via letters. This means playing for an extended period of time, and it provides a lot of freedom that is difficult to find in traditional larps. Much like in tabletop roleplaying the characters are not restricted by the players’ abilities, looks, genders, ages, etc. Furthermore it is easily accessible for anyone who might otherwise have a difficulty attending traditional larps because of children, disabilities, mental illness etc. It also happens to be a wonderful way of brightening up otherwise dull periods, such as the dark winter, or simply to bring some spice and amusement to everyday life. The wonderful feeling of opening your mail box to find beautiful handwritten letters is something we think more people should have the opportunity to experience.
So basicly De Profundis or Quill with a centralised GM or GM team providing a bit of setting and maybe some kicker events (Quill does this with specific guidance, because its pitched as a one-shot, solo rpg).
The website notes that they consider it larp for basicly historical reasons: the first games run like this in Nordic larp culture were labelled larps, and so it stuck. Whether it is or whether it is larpadjacent I guess depends on whether you think you can larp alone (classicly, while locked in a dark closet). If the game is about the characters writing letters, and you sit there in character (and optionally, in costume) writing a letter with appropriate implements and in the appropriate style, you’re certainly “doing what your character is doing”, at least as much as in a live action online game with diagetic video.
But at the least, its useful for thinking about the role IC, diagetic letters can play in a game, and I can see some genres (e.g. regency) where they could be a huge part of play when coupled with physical events.