[Dovecot] Custom Headers/Flags Support in dovecot

I am currently trying to evaluate if I can use dovecot to be used for my imap server like application. This is not an email application but a generic storage server that uses IMAP protocol to save and retrieve messages.

The messages have standard From/To/Date headers and a lot of custom headers for threading conversations e.t.c. As a part of my application requirements I also need to support custom flags and have ability to set/change/retrieve them via IMAP.

1. Does Dovecot support custom flags to be set/changed/retrieved?
2. I understand dovecot optimizes standard RFC822 headers and flags in cache/index files for faster lookups and searches. Does custom headers and flags fall into this optimization algorithm?
3. I am planning to support multiple users and each mailbox may have upto 1000 folders. Each folder may have an average of 20 MIME encoded messages. I am planning to use local storage with single-dbox format. Do you see any issues with this?

thanks Ramesh