Ideas needed for archiving

[Ben]

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