I am attempting to invent Snail Mail Mafia. I think it can be done. All parties in a set group draw for Townspeople/Mafia. The Mafia then sends out letters to all participants, with one letter delivering The Black Spot to her intended victim. Letters which do not contain a Black Spot contain a unique clue/riddle about the identity of the Mafia. Townspeople may choose not to open their letter (saving them from a potential Black Spot), but this also precludes them from their clue as to the murderer's identity.

Participants receive their letters in the mail and then wait until a predetermined day to open them live, as a group, via Skype/Google Hangout*. Townspeople may choose to share their clues with each other or keep them secret. All clues about the Mafia are truthful, except for the one the Mafia gives herself. If there is a murder, that person is exempt from winning but may still speculate as to the killer's identity. After a healthy round of debate and evidence gathering, votes shall be cast to "kill" the suspected Mafia. If wrong, another round follows the next week/month. 

*Though this does run contrary to the mystique of a letter-driven murder mystery. Boo. I'm still chewing on an alternative.