Replication double check

[Blake Hudson]

I'm looking to create a tool that will double check replication,
backups, and migrations to ensure that they are working correctly and no
corruption has crept in.

The tool MUST:
    1) Work via IMAP
    2) Compare a single user's mailbox between 2 servers and report any
differences between the two (this is not a one way tool)

The tool SHOULD:
    1) Perform a comparison of the folder list, subscriptions, and ACLs
    2) Compare the message list, message sizes, and seen state of
messages in each folder, recursively
    3) Compare individual messages for differences (comparing a checksum
of fetched messages is fine). Ideally, i would like to have the option
to perform a full check of all messages or perhaps check just the inbox
or a random selection of messages.
    4) Anything else?

Before I dig in, I searched for existing tools and found none that did
exactly what I want. imapsync is close, but it doesn't really do what
I'm looking for, nor is its output suitable for such a task. imapdump
(part of the imap_tools suite) could be used as a base.

I know Bron had mentioned that he uses a tool internally to check the
consistency of mailboxes across replicated servers. Is there an open
tool that can do this or does anyone (including Bron) want to share
theirs? Does anyone else besides Bron/Fastmail confirm that
replication/backup schemes are not silently corrupting messages?

Thanks in advance,
--Blake