Need advice: mailbox-based \Seen flag

[Boris Lytochkin]

>>> - When a mailbox is opened that has this annotation enabled, we open the
>>> seen state database for the special 'anyone' user instead of the 
>>> authorized user.  This would allow using the same API with minimal 
>>> changes.  Perhaps we could come up with a better userid then 'anyone' 
>>> but its already reserved as special via ACL.
>>
>> It will work as long as we want to use shared folder for every user.
>> As soon as we want to restrict access to a set of users, this would not
>> work.
>
> That's what the 's' right is for.  Only give 's' to those users that you
> want to be able to mark messages as seen.

Ah, 'anyone' will be used only to store per-mailbox seen records.
(Sorry for misunderstanding, I was still sleeping at the moment. :-)
Where to store this file? Again, what about replication?