Yes I wrote something similar, presenting a JSON API on the folders in ~/Maildir for all the local users of a server.

It only does the basic things:

  * Listing folders/Maildirs.
  * Retrieving messages from a named folder.

I didn't have a compelling use for it, but I was curious about whether it would make sense to base my mail-client on an API rather than the filesystem. (In the end my mail-client only reads/writes to ~/Maildir and I avoided the API step in the middle.)