What you could do is find a mail provider that will allow you to create shared mailboxes (Courier and Cyrus IMAP do, for instance), and use an email client that will allow you to manage folder subscriptions.
This way, you can archive all of your old messages in a folder that is accessible to the others in your department, yet you can hide it from your listing when you aren't interested in it or don't want it to be included in searches.
OS X Mail in Leopard will manage folder subscriptions, Thunderbird does now.
There are a number of tools that will manage these files outside of your email, are any of these an option?