I hang out with certain fetish-y types here on DA, and I've occasionally suggested The Letter Game as a variant version of (e.g.) on-line tickling RP. There's been very little interest.
Now I've found this group, and it looks like there's very little interest in general, and not just among fetish-y types. At a guess, I'd say that most people find letter-writing to be "hard" and so producing a series of letters is too much work for too little reward.
also, could these be posted as a picture taken of said longhand letter? (assuming i can get the camera on my phone to do anywhere near a decent job) - the handwriting can be a nice factor