This actually looks really cool (at least when reading it)!
I was wondering if I should do something similar but for a different, perhaps slightly more forward-thinking purpose: JMAP. No idea if the WebSockets part can be shoved through FastCGI (I doubt it) or if it's better to just have it as a daemon.
Just additional food for thought, as IMAP is still very useful and will probably be around for a while, but if you wanna take a stab at that as well then maybe that would also be nice :) (and I'd be more willing to test, as currently I'm using Cyrus exactly for its JMAP support)
Might I ask what is the storage model for this daemon?