Ideas needed for archiving

I've been asked by a user whether it's possible to stop their IMAP client downloading messages older than a given date.

Whilst I'm guessing it may well be possible to do this client-side using folders and subscriptions, it got me thinking .... is there a way to do this server side on dovecot ? i.e. is there a way to "hide" messages from a user without having to archive them off into a seperate account ?

thanks