Just thought to mention:

http://dovecot.procontrol.fi/tmp/README.imapkeywords.html

I'm not sure what to think of it. Good thing in it is that it doesn't limit the number of flags. Bad thing is that I think it's way too slow.

Compared to Dovecot's implementation where I "steal" maildir flags a..z and treat them in the normal maildir way. Bad thing is obviously that it's limited to 26 different flags.

And I really should fix that customflags file soon to be NFS-safe / read-lockless. It's just a bit problematic how to handle removing unused flags when you've hit the 26 limit and try to add new ones.